Bring 4 litres of water to a full rolling boil in your largest pot. Add the 15g of kosher salt — the water should taste assertively of the sea. Add the spaghetti and stir immediately for the first 30 seconds to prevent the strands from sticking together as the starches on their surfaces hydrate and become adhesive in the first moments of cooking. Cook until exactly 2 minutes shy of the package directions' al dente time — the pasta will finish cooking in the skillet during the tossing step and must be significantly underdone when it leaves the boiling water. Before transferring the pasta, reserve at least 240ml of pasta cooking water in a heatproof jug. The pasta water is not optional and not interchangeable with plain water — its dissolved starch content is the emulsifying agent that allows the olive oil to combine with water into a unified, coating sauce rather than remaining as separate, oily droplets that pool at the bottom of the bowl. Reserve it warm and keep it accessible throughout the sauce-making step.

While the pasta cooks, prepare the garlic. Peel all the cloves and slice each one lengthwise into thin, translucent sheets — the thinnest you can produce with a sharp knife, or use a mandoline for the most consistent result. Paper-thin means approximately 1mm or less — thin enough to see light through when held up. This specific thickness is not aesthetic but functional. Uniformly thin garlic slices cook at the same rate throughout their entire cross-section — by the time the outside surface reaches the correct pale golden colour, the inside is equally cooked. Thicker pieces cook unevenly: the exterior browns while the interior remains sharp, raw, and pungent. A single burnt slice among pale golden ones will introduce bitterness into the oil that distributes through the entire sauce — the difference between aglio e olio that tastes fragrant and sweet and aglio e olio that tastes bitter. Take the 3–4 minutes of careful slicing — or use a mandoline — and the cooking step becomes significantly more forgiving.

Approximately 4 minutes before the pasta will be ready for transfer, heat the 120ml of extra-virgin olive oil in your largest skillet over medium-low heat. The skillet size matters — you need enough surface area to eventually toss 450g of spaghetti without it piling above the rim. Add the paper-thin garlic slices and the red pepper flakes simultaneously to the oil before it reaches high heat — adding garlic to already-hot oil causes the thin slices to brown on one side before they can be turned, while cold-oil starting brings the garlic up to temperature slowly and evenly. The red pepper flakes added at this stage bloom their fat-soluble capsaicin and aromatic compounds into the surrounding oil during the 3–4 minute cooking period, producing a pervasive, even warmth throughout the sauce rather than concentrated heat only where a flake is encountered. Cook over medium-low heat, swirling the pan frequently rather than stirring — swirling moves the garlic slices through the oil without breaking the thin slices as a spatula might. The garlic should sizzle very gently — a soft, continuous sizzle rather than an aggressive, spattering fry. Watch every second from the 2-minute mark onward. The target colour is pale champagne gold — very light golden with slightly crisper, darker edges on the thinner end of each slice, but with most of the slice remaining pale. If any slices are browning too quickly, pull the pan off the heat immediately and allow the residual heat to continue the cooking more gently. The difference between pale champagne gold and medium golden brown is approximately 30 seconds at medium-low heat — this is not a step to walk away from.

When the garlic reaches the correct pale gold colour with lightly crisped edges, add 120ml of the reserved hot pasta water to the skillet. The addition will splutter and steam dramatically — this is expected and is not a problem. Swirl or stir vigorously immediately after the water hits the oil. The starch in the hot pasta water begins to combine with the olive oil under the mechanical action of swirling, creating the initial emulsification that produces the cloudy, slightly creamy-looking liquid that will become the sauce's coating base. The transformation from clear oil floating on top of water to a partially unified, opaque emulsion happens quickly with vigorous movement. Keep the heat at medium-low and continue swirling for 30–45 seconds until the liquid looks uniformly cloudy and slightly thickened rather than having distinct oil pools floating on clear water.

Using tongs, transfer the pasta directly from the boiling water to the skillet — do not drain through a colander. The direct transfer with tongs brings a small but beneficial amount of boiling pasta water with the strands, maintaining the sauce's temperature and adding additional starch to the emulsification. Increase the heat to medium-high. Begin tossing the pasta continuously and vigorously — lift from the bottom of the skillet and fold over the top, rotating the pan slightly with each toss to ensure the pasta at the outside of the pan cycles to the centre. The vigorous tossing motion is the emulsification mechanism: the mechanical action breaks the oil into progressively smaller droplets and distributes them evenly throughout the starchy water, producing the glossy, clinging sauce that makes the dish. Toss continuously for 90 seconds, adding more pasta water in 30ml increments whenever the pan looks dry or the pasta begins to stick together. The correct finished consistency is a glossy, lightly creamy coating that clings visibly to every strand — the sauce should not pool oily at the bottom of the skillet and should not look watery. If it looks oily, add more pasta water and toss more vigorously. If it looks watery, continue tossing over higher heat until the emulsion tightens.

Cut the heat completely. Add the roughly chopped fresh parsley if using and toss for 10 seconds — the residual heat wilts the parsley gently, releasing its aromatic oils without cooking it to dark, flavour-depleted softness. Taste for salt and adjust. Divide among four warm bowls immediately — aglio e olio tightens and the oil begins to separate from the starch emulsion as it cools, so the window between leaving the pan and the first bite is short. Drizzle a thin stream of your best finishing extra-virgin olive oil over each bowl — this final raw drizzle adds the fresh, uncooked fruity-peppery character of quality olive oil that the cooking process modifies. Do not add cheese. The dish is complete as it is.