Bring a large pot of salted water to a rolling boil. Add fettuccine or linguine and cook until al dente, about 8 to 10 minutes. Reserve ½ cup of pasta water, drain the pasta, and set aside.

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

Add sliced mushrooms to the skillet and sauté until tender, about 6 to 8 minutes. Stir in minced garlic and cook for another minute.

Pour in heavy cream, stirring constantly. Gradually add shredded Gruyere cheese, mixing until melted. Adjust consistency with reserved pasta water if needed.

Add the drained pasta to the skillet, tossing to coat with sauce. Season with salt and pepper to taste.

Remove from heat and serve immediately, garnished with chopped parsley if desired.