In a large pot, bring salted water to a rolling boil. Add your 12 oz of pasta and cook until al dente, about 8-10 minutes. Once done, reserve ½ cup of the pasta water for later, then drain the pasta and set it aside in a large bowl.

In a skillet over medium heat, melt 2 tablespoons of unsalted butter along with 2 tablespoons of olive oil. Once bubbling, add the sliced leeks. Cook for about 12-15 minutes until caramelized.

Add 8 oz sliced mushrooms to the skillet with the leeks. Cook for 6-8 minutes until softened. Stir in 2 minced cloves of garlic, cooking for another minute until fragrant.

Lower the heat and pour in ½ cup of heavy cream, stirring gently. Gradually add ¾ cup of shredded Gruyere cheese, stirring until melted and smooth.

Add the drained pasta to the skillet with the sauce, tossing gently to coat. If too thick, add reserved pasta water for desired consistency.

Transfer to serving plates and season with salt and pepper. Garnish with freshly chopped parsley if desired.