In a large sauté pan over medium heat, melt 2 tablespoons of olive oil with 2 tablespoons of butter. Add the leeks, salt, and sugar. Cook, stirring occasionally, for about 20 minutes until they’re golden brown and jammy. If needed, add 1–2 tablespoons of water to prevent burning.

Pour in the sherry wine and cook until it has almost fully evaporated, intensifying the leeks’ sweet and savory notes. Transfer to a bowl and set aside.

In the same pan, melt the remaining 1 tablespoon of butter. Spread the oyster mushrooms in a single layer and let them sear undisturbed for 4 minutes to develop that golden edge. Flip and cook the other side until browned. Season lightly with salt and pepper.

Add garlic and sage to the mushrooms and sauté for 1 minute. Then return the leeks to the pan, followed by heavy cream, balsamic vinegar, and lemon zest. Let it simmer for 2–3 minutes until the sauce slightly thickens. Adjust seasoning as needed.

Boil the fettuccine until al dente in salted water. Before draining, reserve 1 cup of the pasta water.

Toss the cooked fettuccine into the sauce with reserved pasta water, Gruyere, and black pepper. Stir well until the cheese melts and the sauce evenly coats the pasta.

Divide into bowls and top with toasted pine nuts for crunch. Serve hot and enjoy immediately.