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

In a large skillet, melt butter and olive oil over medium heat. Add sliced leeks, stirring gently, and cook for 12-15 minutes until golden brown.

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

Reduce heat to low, pour in heavy cream, and add half the Gruyere cheese, stirring continuously until melted and creamy.

Toss the cooked pasta into the skillet, mixing well. Add reserved pasta water to adjust consistency if needed. Stir in remaining Gruyere cheese and season with salt and pepper.

Serve immediately, garnished with chopped parsley.