Cook the Pasta: Bring a large pot of salted water to a boil. Add 12 oz of pasta and cook for 8-10 minutes until al dente. Reserve ½ cup of pasta water before draining.

Caramelize the Leeks: In a skillet, melt 2 tablespoons of unsalted butter and 2 tablespoons of olive oil. Add sliced leeks and cook for 12-15 minutes until golden.

Sauté the Mushrooms and Garlic: Add 8 oz of mushrooms to the skillet and sauté for 6-8 minutes. Stir in minced garlic and cook for another minute.

Create the Cheese Sauce: Reduce heat and add ½ cup heavy cream. Gradually add 1.5 cups of shredded Gruyere cheese, stirring until melted.

Combine Pasta and Sauce: Toss the cooked pasta with the cheese sauce, adding reserved pasta water if needed. Season with salt and pepper.

Serve and Garnish: Serve immediately on warm plates and optionally garnish with chopped parsley.