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.