Begin by bringing a large pot of salted water to a rolling boil. Add 12 ounces of your chosen pasta and cook according to package instructions until al dente, about 8-10 minutes. Reserve ½ cup of the pasta water for later use, then drain the pasta and set it aside.

In a large skillet, melt 2 tablespoons of unsalted butter with 2 tablespoons of olive oil over medium heat. Add the sliced leeks and sauté for about 12-15 minutes, stirring occasionally, until they become golden brown and caramelized.

Stir in 8 ounces of sliced mushrooms. Cook for an additional 6-8 minutes, allowing the mushrooms to soften and brown. Add 2 minced garlic cloves in the last minute.

Reduce the heat to low, then pour in ½ cup of heavy cream and half of the 1½ cups of shredded Gruyere cheese. Stir continuously until the cheese fully melts and the sauce becomes smooth and creamy.

Gently fold the cooked pasta into the creamy sauce, ensuring each strand is coated. If the sauce appears too thick, gradually add a bit of the reserved pasta water until you've achieved the desired consistency. Mix in the remaining Gruyere cheese.

Taste your creamy mushroom Gruyere pasta and season with salt and black pepper to your liking. Serve immediately, garnishing with freshly chopped parsley if desired.