Begin by boiling a large pot of salted water over high heat. Once it reaches a rolling boil, add 12 ounces of fettuccine or linguine and cook for 8-10 minutes, or until al dente. Reserve ½ cup of the starchy pasta water for later use, then drain the rest and set the cooked pasta aside.

In a large skillet, melt 2 tablespoons of unsalted butter with 2 tablespoons of olive oil over medium heat. Add 2 sliced large leeks to the skillet and sauté for 12-15 minutes, stirring occasionally until they are beautifully caramelized and golden brown.

Once the leeks are caramelized, add 8 ounces of sliced cremini or button mushrooms to the skillet. Cook for 6-8 minutes until they become tender, then add 2 minced garlic cloves, cooking for an additional minute.

Lower the heat and pour in ½ cup of heavy cream and add ¾ cup of shredded Gruyere cheese to the skillet. Stir constantly until the cheese is fully melted and the sauce reaches a creamy consistency.

Gently fold the cooked pasta into the cheese sauce, ensuring each strand is coated. If the sauce is too thick, gradually add reserved pasta water until you achieve your desired consistency.

Plate the creamy mushroom Gruyere pasta immediately. Sprinkle chopped fresh parsley on top for a touch of freshness before serving.