Begin by bringing a large pot of salted water to a rolling boil over high heat. Add 12 oz of fettuccine or linguine and cook until al dente, about 8-10 minutes. Reserve ½ cup of pasta water, then drain and set aside.

In a large skillet, heat 2 tablespoons of unsalted butter and 2 tablespoons of olive oil over medium heat until melted. Add sliced leeks and sauté for about 12-15 minutes until golden brown.

Increase the heat slightly and stir in 8 oz of sliced mushrooms. Cook for 6-8 minutes until tender, then add minced garlic and cook for an additional minute.

Reduce heat to low and pour in ½ cup of heavy cream, stirring gently. Gradually add ¾ cup of shredded Gruyere cheese, stirring until melted and smooth.

Add the drained pasta to the sauce, tossing gently. If too thick, add reserved pasta water. Fold in remaining ¾ cup of cheese and season with salt and pepper.

Transfer to serving plates and sprinkle with chopped parsley, if desired. Serve immediately.