Bring a large pot of salted water to a boil. Add pasta and cook for 8–10 minutes until al dente. Reserve ½ cup pasta water, then drain.

In a large skillet, heat butter and olive oil over medium heat. Add sliced leeks and sauté for 12–15 minutes until golden brown.

Stir in sliced mushrooms and cook for 6–8 minutes. Add minced garlic and sauté for an additional minute until fragrant.

Lower heat and add heavy cream. Gradually stir in shredded Gruyere cheese until melted and creamy.

Toss the cooked pasta in the cheese sauce. If sauce is too thick, add reserved pasta water until desired consistency is reached. Season with salt and pepper.

Serve immediately and garnish with parsley if desired.