Cook the pasta in a large pot of salted boiling water until al dente. Reserve 1/2 cup of pasta water, drain, and set aside.

In a skillet, melt butter and heat olive oil. Add sliced leeks, cooking for 12-15 minutes until caramelized.

Stir in sliced mushrooms and cook for 6-8 minutes. Add minced garlic and cook for 1 minute until fragrant.

Reduce heat and add heavy cream and half of the Gruyere cheese. Stir until melted and creamy.

Add cooked pasta to the sauce, mixing until coated. Adjust sauce consistency with reserved pasta water. Stir in remaining Gruyere cheese, salt, and pepper.

Serve immediately, garnished with chopped parsley if desired.