Clean leeks perfectly: trim roots and dark greens. Halve lengthwise, slice 5 mm thick, place in large bowl of cold water. Swish vigorously, let grit sink, lift out. Repeat twice. Drain well.

Bring 6 quarts water to a rolling boil. Salt aggressively (3 tbsp).

Heat your widest sauté pan (14-inch) over medium-low. Add 50 g butter. When melted, add ALL leeks with a pinch of salt. Cook gently 10–12 minutes, stirring occasionally, until completely soft and sweet (no color).

Increase heat to high. Add remaining 30 g butter + ALL mushrooms. Cook undisturbed 5 minutes until golden underneath. Toss, cook another 4–5 minutes until deeply browned. Add garlic + thyme in last 60 seconds.

Drop fettuccine into boiling water. Set timer for exactly 8 minutes.

Pour white wine into mushroom pan — dramatic sizzle! Scrape up every brown bit and reduce by two-thirds (2–3 minutes).

Remove pan from heat completely. Pour in cold cream and stir. Return to medium-low and simmer gently 2 minutes.

Reserve 2 full cups pasta water. Using tongs, transfer pasta directly into sauce (some water clinging is perfect). Add ¾ cup pasta water and ALL three cheeses in 4 stages while tossing vigorously off heat. Sauce will go from watery → creamy → glossy silk. Add more pasta water if needed.

Remove from heat again. Add lemon zest, juice, parsley, tons of cracked pepper, and one final 1 tsp raw butter. Toss gently. Let rest in warm pan 5 minutes — sauce thickens perfectly.

Twirl into warmed bowls, shower with extra Parmesan, more pepper, and a few drops of truffle oil if using. Serve immediately with chilled Pinot Grigio and zero conversation for the first two minutes.