Fill a large pot with water, add a generous pinch of salt, and bring it to a rolling boil over high heat. Add pasta and cook until al dente, approximately 8–10 minutes. Reserve 1/2 cup of pasta water before draining.

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

Add sliced mushrooms to the skillet and sauté for 6–8 minutes until soft and browned. Toss in minced garlic and cook for an additional minute until fragrant.

Lower heat to a gentle simmer, pour in heavy cream, and gradually add half of the shredded Gruyere cheese, stirring until melted and creamy, about 3–5 minutes.

Carefully add the drained pasta into the skillet with the cheese sauce. Toss to coat with the creamy sauce and mix in the remaining Gruyere cheese. Adjust with reserved pasta water if too thick.

Plate the pasta and garnish with chopped fresh parsley if desired. Serve immediately.