Bring a large pot of salted water to a rolling boil. Add fettuccine or linguine and cook until al dente (8-10 minutes). Reserve ½ cup pasta water, drain, and keep the pasta warm.

In a large skillet, heat unsalted butter and olive oil over medium heat. Add sliced leeks and cook for 12-15 minutes until golden brown and tender.

Add sliced mushrooms to the skillet and sauté for 6-8 minutes until softened. Then, add minced garlic and cook for another minute.

Reduce heat to low, pour in heavy cream and half of the shredded Gruyere cheese. Stir continuously until the cheese melts and the sauce is creamy.

Add the cooked pasta into the skillet, tossing to coat evenly. Use reserved pasta water if the sauce is too thick. Stir in the remaining Gruyere and season with salt and pepper.

Plate the pasta immediately and sprinkle with chopped parsley, if desired.