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

In a large skillet, combine 2 tablespoons of unsalted butter and 2 tablespoons of olive oil over medium heat. Once melted, add the sliced leeks and sauté for 12-15 minutes until golden brown and fragrant.

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

Lower the heat and gradually pour in ½ cup of heavy cream. Continuously stir while adding ¾ cup of shredded Gruyere cheese until it melts and forms a creamy sauce.

Add the cooked pasta to the skillet, tossing gently to coat every strand in the sauce. Add reserved pasta water if the sauce is too thick. Stir in the remaining ¾ cup of Gruyere cheese, along with salt and pepper to taste.

Transfer the creamy Mushroom Gruyere Pasta to serving plates. Garnish with freshly chopped parsley and serve immediately.