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

In a large skillet, melt the butter and add the olive oil over medium heat. Add the sliced leeks and sauté for 12-15 minutes until caramelized.

Add the sliced mushrooms to the skillet and cook for about 6-8 minutes until browned. Stir in the minced garlic and cook for an additional minute.

Pour in the heavy cream and half of the Gruyere cheese into the skillet. Stir continuously until the cheese melts into a velvety sauce.

Add the drained pasta to the skillet, tossing to combine. If the sauce is too thick, gradually add reserved pasta water until desired consistency is reached. Stir in remaining cheese, season with salt and pepper.

Serve immediately with optional parsley garnish.