In a large skillet over medium heat, add olive oil and butter. Once melted, add the sliced leeks, sprinkle with salt and sugar, and sauté for about 20 minutes until golden brown.

Pour in the sherry wine and stir well, allowing it to simmer for 2-3 minutes until it reduces. Set the leeks aside.

In the same pan, add remaining butter and sauté the oyster mushrooms for about 4 minutes. Season with salt and black pepper.

Add minced garlic and chopped sage leaves to the mushrooms and stir for 1 minute. Return the caramelized leeks, then pour in the heavy cream, balsamic vinegar, and lemon zest, simmer for 2-3 minutes.

Cook the fettuccine in salted boiling water according to package instructions. Reserve 1 cup of pasta water before draining.

Combine the drained fettuccine with the sauce over low heat, adding reserved pasta water as needed. Stir in the grated Gruyere and season with black pepper.

Serve garnished with toasted pine nuts.