Bring a large pot of water to a full rolling boil and add the 6g of salt. Add the pappardelle or fettuccine and cook until exactly 2 minutes shy of the package's al dente time. The wide, flat format of pappardelle is the specific pasta choice for this sauce — its large surface area maximises contact with the creamy mushroom coating, and its slight thickness holds up through the finishing step in the sauce without becoming soft. Fettuccine is an equally valid narrower alternative with a comparable surface-area advantage over spaghetti or short pasta. Before draining, reserve 240ml of the starchy pasta water in a heatproof jug and keep it warm throughout. Drain without rinsing — the surface starch is an emulsification component in the final sauce.

Heat a large, deep skillet over medium-high heat until properly hot — a drop of water should evaporate immediately on contact. Add 30ml of the olive oil and immediately add approximately half the 500g of sliced mixed mushrooms in a single, even layer with space between each piece. This single-layer, uncrowded placement is the technique prerequisite for proper mushroom searing. Crowded mushrooms — stacked on top of each other or pressed together — create a sealed steam environment: the moisture released from each mushroom is trapped against its neighbours, the pan temperature drops dramatically, and the mushrooms steam in their own liquid rather than caramelising at the pan surface. The result is pale, wet, soft mushrooms rather than the deeply golden, slightly firm, caramelised mushrooms that provide the dish's textural contrast. With space between pieces, the released moisture evaporates immediately into the open air above the pan and the mushroom's surface makes direct contact with the hot oil and pan — the correct conditions for Maillard reaction browning. Leave completely undisturbed for 3–4 minutes. Do not stir, do not shake the pan. After 3–4 minutes the underside of each piece should show deep golden-brown colour. Toss once and cook for another 2 minutes until the second side shows comparable colour. Transfer to a plate. Add the remaining 15ml of olive oil and repeat with the second batch. These caramelised mushrooms are the textural element of the finished dish — they are set aside now and returned to the pasta at the final step, maintaining their seared character rather than softening into the sauce during the subsequent cooking stages.

Reduce the heat to medium and add 40g of the butter to the same skillet without cleaning it — the fond from the mushroom sear contributes flavour to everything subsequently cooked in the pan. Add the 200g of finely chopped white button mushrooms and the 2g of salt. Cook, stirring frequently, for 8–10 minutes. This is the most patience-dependent and most flavour-generative step in the recipe. The white button mushrooms at this stage follow a predictable three-phase cooking arc: first they release their moisture into the pan in a pool of liquid that makes the pan look wet and fully liquid; then the liquid progressively evaporates as cooking continues, the pan becoming increasingly dry and the mushrooms beginning to colour; finally, when all the moisture has evaporated, the mushrooms begin to fry in the residual butter fat and their cell walls fully collapse into a dense, concentrated, paste-like mass. By the time all liquid has evaporated and the mushrooms have reached this paste-like, golden state at the 8–10 minute mark, they contain all the concentrated umami compounds of 200g of raw mushrooms compressed into a small volume of intensely flavoured mass. This base dissolves almost invisibly into the cream sauce and provides the depth that makes the finished sauce taste more complex than its ingredient list explains. Add the thinly sliced garlic, fresh thyme leaves, and finely minced rosemary. Cook, stirring constantly, for 90 seconds until the garlic is fragrant and beginning to show very faint golden colour at the edges — the thyme and rosemary bloom their aromatic compounds into the mushroom-and-butter fat simultaneously.

Pour in the 120ml of dry white wine and increase the heat to medium-high. The wine immediately sizzles and loosens the accumulated fond from the mushroom sear and the mushroom base from the pan surface — stir firmly to dissolve every bit into the wine. Allow to simmer vigorously for 2 minutes until the wine has reduced by approximately half and its sharp, raw alcohol edge has completely cooked off — the remaining wine should smell of concentrated fruit rather than raw spirit. The white wine provides the secondary acid note that prevents the cream sauce from tasting flat and one-dimensionally rich. Reduce the heat to medium-low and add the 240ml of heavy cream and the grated nutmeg. Stir to combine and bring to a very gentle simmer. Cook for 3 minutes, stirring occasionally, until the sauce has thickened enough to coat the back of a spoon — a line drawn through the sauce on the spoon's surface should hold its edges rather than flowing back immediately. The nutmeg is the classic French and Italian seasoning for cream sauces containing mushrooms — at ¼ tsp it is not detectable as nutmeg but adds the characteristic warm, slightly sweet-spiced depth that makes cream mushroom sauces taste more complex.

Add the drained, underdone pasta directly to the cream sauce in the skillet along with 180ml of the reserved pasta water. Increase the heat to medium. Toss continuously with tongs for 2–3 minutes — lifting the pasta from the bottom and folding it over the top in a constant circular motion. The pasta finishes cooking during this tossing period, absorbing the mushroom cream sauce's flavour directly into its structure. The surface starch from the pasta and the starch already dissolved in the pasta water combine with the cream's fat to produce the progressive thickening and emulsification that makes the sauce cling to each strand rather than pooling at the bottom. The sauce should be visibly coating the pasta after 2–3 minutes — glossy, slightly thickened, and flowing when the pan is tilted rather than dry or stiff.

Remove the pan from the heat completely. Add the remaining 20g of butter and toss until melted and incorporated — the cold butter disperses into the warm sauce as fine droplets, creating the smooth, emulsified finish that makes the sauce glossy rather than greasy. Add the 100g of Parmigiano-Reggiano in two or three additions, tossing vigorously between each addition. The off-heat addition prevents the proteins from seizing and ensures smooth melting into the sauce. Add the lemon zest and toss to distribute. Add the 3g of freshly cracked black pepper. If the sauce has tightened beyond the correct flowing consistency, add the remaining pasta water in 30ml increments, tossing after each addition. Fold in the reserved caramelised mushroom slices gently — a lifting and folding motion rather than vigorous stirring, which would break the seared mushrooms into small pieces rather than preserving them as identifiable, texturally distinct pieces distributed throughout the pasta. Taste and adjust with additional salt if needed.

Divide among four warm bowls immediately. Scatter the 20g of roughly chopped fresh parsley over each bowl — its clean, slightly bitter herbal freshness provides the aromatic counterpoint that the richness of the cream and mushrooms specifically needs. Scatter additional finely grated Parmigiano-Reggiano over each portion. Serve immediately while the pasta is at maximum creaminess.