Heat olive oil and 2 tablespoons butter in a large sauté pan over medium heat. Add the sliced leeks, salt, and sugar. Cook for 20 minutes, stirring occasionally, until golden and caramelized. If the pan gets dry, add a tablespoon or two of water.

Deglaze the pan with sherry wine and let it reduce for 2-3 minutes. Transfer the leeks to a bowl and set aside.

In the same pan, melt the remaining butter. Add the mushrooms in a single layer and let them brown undisturbed for about 4 minutes per side. Season with salt and pepper.

Add minced garlic and sage, cooking for 1 minute until fragrant. Return leeks to the pan and stir in the heavy cream, balsamic vinegar, and lemon zest. Simmer for 2-3 minutes until thickened.

Meanwhile, cook the fettuccine until al dente. Reserve 1 cup of pasta water before draining.

Add the pasta to the sauce along with the reserved pasta water, Gruyere, and black pepper. Toss to coat and simmer for 2 minutes until the cheese melts and the sauce becomes creamy.

Serve hot, topped with toasted pine nuts and a sprinkle of extra cheese if desired.