• Heat 2 tablespoons olive oil and 2 tablespoons butter in a large sauté pan over medium heat. Add sliced leeks and season with ½ teaspoon salt and ½ teaspoon sugar. Cook until golden and caramelized, about 20 minutes, stirring occasionally. If the pan gets too dry, add 1-2 tablespoons of water.
• Add sherry and continue to cook until the wine has reduced. Remove leeks from the pan and set them aside.
• In the same pan, melt another tablespoon of butter. Spread mushrooms in an even layer, giving them plenty of space in between. Let them sit until browned, about 4 minutes. Flip and continue to brown on the other side. Season with salt and pepper.
• Add minced garlic and sage and sauté for 1 minute. Add back leeks along with ¾ cup heavy cream, balsamic vinegar, and lemon zest. Bring to a simmer and continue to cook until thickened, about 2-3 minutes. Season the sauce with salt and pepper.
• Meanwhile, cook fettuccine until al dente. Reserve 1 cup of the cooking pasta water.
• Add fettuccine, 1 cup reserved pasta water, gruyere, and black pepper to the sauce, tossing to coat. Simmer for 2 minutes or until cheese is melted and the pasta is well coated. Give a final seasoning with salt and pepper.
• Portion pasta into 4 bowls and garnish with toasted pine nuts. Serve immediately.

Notes:

• Although I used oyster mushrooms, you can substitute them with another type of mushroom such as cremini, button, chanterelle, or a mixture of mushrooms.
• If preparing the dish ahead of time, make the sauce following steps 1-4. Reheat it when ready to toss with the pasta.