Bring a large pot of water to a full rolling boil and add the 6g of kosher salt. Add the fettuccine or linguine and cook according to the package directions until al dente — approximately 8–9 minutes. Before draining, reserve 240ml of the starchy pasta water in a heatproof jug — always reserve more than you expect to need, as the crème fraîche sauce can tighten unpredictably during the pasta combining step and the reserved water is the only available correction tool. Drain without rinsing and set aside.

This step requires a separate non-stick skillet rather than the sauce pan — salmon's delicate structure makes it prone to sticking to stainless steel, and the non-stick surface allows the cubes to be turned without breaking. Pat the 500g of salmon cubes completely dry on all sides with paper towels — thorough drying is the prerequisite for a proper golden sear rather than steaming grey surfaces. Season evenly with the 1g of salt and 1g of black pepper. Heat the non-stick skillet over medium-high heat and add the 15g of butter. Allow the butter to melt and its foam to subside — the subsiding foam indicates the water has evaporated and the fat is at the correct temperature for searing without burning. Add the salmon cubes in a single layer without crowding — crowded salmon steams rather than sears. Sear for 1.5–2 minutes per side until golden on the exterior. The critical internal temperature is 52°C — at this point the salmon is just cooked through at the surface while retaining a slightly translucent, slightly pink centre that the residual heat will finish to a tender, moist doneness as it rests. Pulling salmon at 52°C produces the specific texture — firm enough to hold its cube shape when folded into the pasta, still moist and yielding at the centre — that overcooked salmon at 65°C+ cannot provide. Transfer immediately to a plate and set aside.

In a large, deep skillet over medium heat, melt the 25g of butter with the 15ml of olive oil together. The combined fat provides both the butter's sweet dairy richness for the shallots and the olive oil's temperature moderation that prevents the butter from browning during the 3–4 minutes of shallot cooking. Add the 90g of finely minced shallots. Cook for 3–4 minutes, stirring frequently, until completely softened and translucent — shallots at this point should have no raw crunch remaining and their natural sugars should have begun developing into mild sweetness. Add the 16g of minced garlic and the tied bundle of dill stems simultaneously. Cook for 1 minute, stirring continuously — the garlic blooms its aromatic compounds into the surrounding fat and the dill stems begin releasing their aromatic oils into the hot shallot base. Increase the heat to medium-high and pour in the 120ml of dry white wine. Allow to bubble vigorously for 2–3 minutes, scraping the bottom of the pan to lift any fond. The wine reduces by approximately half, its sharp alcohol edge cooking off and leaving its fruity acidity and depth. The acid of the white wine specifically bridges the cream's richness and the lemon's brightness — producing a sauce that tastes more complete than cream and lemon without wine would.

Reduce the heat to medium-low. Add the 200ml of heavy cream, 150ml of whole milk, and 120g of crème fraîche to the skillet. Stir continuously until the crème fraîche dissolves completely into the cream and milk — it resists smooth integration if added to very hot liquid without stirring, potentially producing visible curds rather than a uniform sauce. The three-dairy combination is calibrated for this specific dish: heavy cream provides richness and coating quality; milk lightens the cream's fat content so the sauce flows freely rather than coating oppressively; crème fraîche provides the specific slightly tangy, slightly sour note that differentiates this from a plain cream sauce and that has a natural, deeply classical affinity with both dill and salmon. Add the lemon zest and bring to a gentle simmer. Simmer for 4–5 minutes, stirring occasionally, until the sauce has thickened enough to coat the back of a spoon — a clear line held when a finger is drawn through the coating. Remove and discard the tied dill stems — they have given their aromatic contribution during the simmer and their woody texture should not appear in the finished sauce.

Add the drained fettuccine to the cream sauce along with 120ml of the reserved pasta water. Toss vigorously with tongs for 1–2 minutes — the pasta's surface starch releases into the sauce during the tossing, combining with the cream's fat to produce the glossy, slightly emulsified coating that distinguishes pasta finished in a sauce from pasta simply plated with sauce poured over it. Add the 60g of baby spinach and toss for approximately 30 seconds until just wilted and bright green — the spinach should wilt completely in the residual heat of the sauce without any additional time on the burner, preserving its vivid colour and preventing the waterlogged dark green of overcooked spinach.

Remove the skillet from the heat completely. This is the most care-requiring step of the recipe — the salmon is fragile and the lemon juice must be added off the heat to prevent it from curdling the crème fraîche. Gently fold in the seared salmon cubes — a lifting and turning motion rather than vigorous stirring, which would break the cubes into flakes rather than maintaining their distinct, textural 2.5cm pieces. Add the 20g of chopped fresh dill — the main fresh dill addition that provides the vivid, aromatic, clean herbal character the dish is named for and that the stem infusion alone cannot provide. Add the 50ml of lemon juice — the bright, clean acid that cuts through the cream's richness and amplifies the dill's aromatic freshness. Add the 40g of finely grated Parmesan, the 1g of salt, and the 2g of black pepper. Fold gently once more to distribute. The sauce at this point should taste bright, creamy, herbaceous, and balanced — the dill prominent and fresh, the lemon vivid rather than simply acidic, the cream rich but not heavy. If the sauce has tightened beyond the correct flowing, coating consistency, add the remaining pasta water in 30ml increments. Divide among four warm shallow bowls immediately. Garnish each bowl with fresh dill fronds, a small pinch of additional lemon zest, and optional red pepper flakes. Serve without delay.