Bring a large pot of salted water to a rolling boil over high heat. Add 12 ounces of fettuccine or linguine and cook until al dente, about 8 to 10 minutes. Reserve ½ cup of pasta water for later use, then drain the pasta and set it aside in a large bowl.

In a large skillet, heat 2 tablespoons of unsalted butter and 2 tablespoons of olive oil over medium heat. Add the sliced leeks and cook for 12 to 15 minutes until golden brown and caramelized.

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

Reduce the heat to low and pour in ½ cup of heavy cream, stirring to combine. Gradually add ¾ cup of the shredded Gruyere cheese, mixing until everything is melted and creamy.

Add the drained pasta to the skillet, tossing it gently into the sauce. If too thick, add reserved pasta water a little at a time. Mix in the remaining Gruyere cheese and season with salt and pepper.

Serve the creamy mushroom Gruyere pasta immediately, garnishing with freshly chopped parsley.