Begin by bringing a large pot of salted water to a rolling boil over high heat. Once boiling, add 12 oz of your preferred pasta and cook until al dente, about 8-10 minutes. Reserve ½ cup of pasta water, then drain the pasta and set aside.

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

Once the leeks are caramelized, add 8 oz of sliced mushrooms and sauté for 6-8 minutes until browned and tender. Stir in 2 minced garlic cloves and cook for an additional minute.

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

Add the cooked pasta to the skillet, tossing to coat. If too thick, gradually stir in reserved pasta water until desired consistency. Incorporate remaining ¾ cup of Gruyere cheese and season with salt and pepper.

Serve immediately, garnishing with freshly chopped parsley if desired.