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

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

Add sliced mushrooms to the skillet and cook for 6-8 minutes until colored. Stir in minced garlic and sauté for an additional minute until fragrant.

Lower heat to medium-low and pour in heavy cream. Add shredded Gruyere and stir until melted and creamy, about 3-4 minutes.

Combine the cooked pasta with the creamy sauce, adding reserved pasta water to reach desired consistency. Stir in remaining Gruyere and season with salt and pepper.

Serve hot, garnished with fresh parsley for color and freshness.