Begin by heating the olive oil and butter in a large sauté pan over medium heat. Add the sliced leeks along with a pinch of salt and a sprinkle of sugar. Cook for about 20 minutes until golden brown and caramelized, then stir in sherry wine and reduce slightly before removing the leeks from the pan.

In the same sauté pan, add another tablespoon of butter and let it melt over medium-high heat. Add the oyster mushrooms, cooking for about 5-7 minutes until browned and tender. Incorporate minced garlic and sage leaves, stirring for an additional minute.

Return the caramelized leeks to the pan with the mushrooms, stir to combine, and pour in heavy cream, balsamic vinegar, and lemon zest. Reduce heat to low, simmer for 2-3 minutes while stirring, then season with salt and pepper.

While the sauce simmers, bring a pot of salted water to a boil and cook fettuccine until al dente, about 8-10 minutes. Reserve 1 cup of the cooking water, drain the pasta, and set aside.

Add the cooked fettuccine to the sauté pan with the sauce. Mix in reserved pasta water to achieve desired consistency, then stir in grated Gruyere cheese and black pepper. Cook for 2 more minutes until the cheese melts.

Divide the pasta into serving bowls, garnishing with toasted pine nuts. Serve immediately.