In a large pot, bring salted water to a rolling boil over high heat. Add the pasta and cook for about 8-10 minutes until al dente. Reserve ½ cup of the starchy pasta water, then drain the pasta and set it aside in a colander.

Heat a skillet over medium heat and add butter and olive oil. Once melted, add the sliced leeks and sauté for 12-15 minutes until golden brown.

Stir in the sliced mushrooms and cook for 6-8 minutes, allowing them to brown and release moisture. Then, add minced garlic and stir for an additional minute.

Lower the heat and pour in heavy cream, stirring well. Gradually add shredded Gruyere cheese, continuing to stir until melted and creamy.

Add the drained pasta into the pan with the creamy sauce. Toss until thoroughly coated, adding reserved pasta water if necessary. Season with salt and pepper to taste.

Serve immediately, garnished with freshly chopped parsley if desired.