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

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

Add sliced mushrooms to the skillet and cook for 6-8 minutes until tender. Stir in minced garlic and cook for 1 more minute.

Pour in heavy cream, stirring to combine it with leeks and mushrooms. Gradually add shredded Gruyere cheese, mixing until melted and creamy.

Add drained pasta to the skillet, tossing to coat in the sauce. If too thick, incorporate reserved pasta water until desired consistency is reached.

Remove from heat, plate the pasta, and garnish with fresh parsley if desired. Serve immediately.