In a large sauté pan, heat 2 tablespoons of olive oil and 2 tablespoons of butter over medium heat. Add the sliced leeks, 1 teaspoon of salt, and 1 teaspoon of sugar, cooking for about 20 minutes until golden brown.

Pour ¼ cup of sherry wine into the pan with the caramelized leeks and cook for an additional 2-3 minutes until reduced.

Remove the leeks and melt another tablespoon of butter in the same pan. Add the oyster mushrooms in a single layer, cooking for about 4 minutes on each side.

Once the mushrooms are browned, add 2 minced garlic cloves and a handful of sage leaves. Sauté for about 1 minute until fragrant.

Lower the heat and pour in 1 cup of heavy cream, 1 tablespoon of balsamic vinegar, and the zest of half a lemon. Simmer for 2-3 minutes until thickened.

While the sauce simmers, bring salted water to a boil. Cook 12 ounces of fettuccine until al dente, reserving 1 cup of pasta water before draining.

Add drained fettuccine into the sauce along with reserved pasta water. Stir in 1 cup of grated gruyere cheese until melted.

Plate the pasta and sprinkle with toasted pine nuts, serve immediately.