01 -
Warm a large sauté pan with olive oil and 2 tablespoons of butter at medium heat. Toss in the leeks, sprinkle with sugar and salt, and stir now and then. Let them cook until they become golden and soft, about 20 minutes. If needed, splash in a bit of water to keep the pan from getting too dry.
02 -
Stir in the sherry and cook down the liquid until it shrinks. Scoop the leeks from the pan and set them aside for later.
03 -
Use the same pan and melt the last spoonful of butter. Lay the mushrooms flat in the pan with space between them. Don’t touch them for 4 minutes so they get brown, then turn them to brown the other side. Sprinkle with a bit of salt and black pepper.
04 -
Add the chopped garlic and sage leaves into the pan with the mushrooms. Cook briefly for 1 minute. Return the saved leeks to the pan and mix in the heavy cream, balsamic, and lemon zest. Let this simmer until it thickens, which should take 2-3 minutes. Taste and add more salt or pepper if needed.
05 -
While the sauce thickens, boil the fettuccine until it’s just tender but not mushy. Keep back a cup of the starchy pasta water before draining.
06 -
Stir the hot noodles into the sauce along with the reserved pasta water, Gruyere, and black pepper. Mix and heat for about 2 minutes. The cheese should melt and coat the pasta well. Season again if needed.
07 -
Scoop the pasta into 4 dishes and sprinkle with toasted nuts on top. Eat it while it's warm.