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

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

Add 8 oz of sliced mushrooms to the skillet. Cook for about 6 to 8 minutes until tender. Stir in 2 minced garlic cloves and sauté for an additional minute.

Lower the heat to medium-low and pour in ½ cup of heavy cream, stirring gently. Gradually add ¾ cup of shredded Gruyere cheese, mixing until melted and creamy.

Add the cooked pasta to the skillet, tossing gently to coat. Stir in reserved pasta water if the sauce is too thick, then mix in the remaining Gruyere cheese, seasoning with salt and pepper.

Divide the pasta into serving bowls and garnish with chopped parsley if desired. Serve immediately.