In a large sauté pan, heat olive oil and butter over medium heat until melted. Add sliced leeks, salt, and sugar. Cook for about 20 minutes until golden brown and caramelized.

Pour in the sherry wine, deglazing the pan by scraping up browned bits. Allow to reduce for about 3-5 minutes until almost evaporated.

In the same pan, melt remaining butter over medium-high heat. Add oyster mushrooms, season with salt and pepper, and sauté for approximately 8 minutes until browned.

Add minced garlic and torn sage leaves to the mushrooms and sauté for 1 minute until fragrant. Return the caramelized leeks to the pan, stirring to combine.

Pour in the heavy cream, balsamic vinegar, and lemon zest, stirring to combine. Simmer for 2-3 minutes until slightly thickened.

Cook fettuccine in salted boiling water until al dente, reserving 1 cup of pasta water before draining.

Add drained fettuccine to the sauce, pouring in reserved pasta water and grated gruyere cheese. Toss over low heat for 2 minutes until well-coated.

Divide into serving bowls and garnish with toasted pine nuts. Serve immediately.