Bring a large pot of salted water to a rolling boil. Add 12 oz of fettuccine or linguine and cook as per package instructions until al dente, about 8-10 minutes. Reserve ½ cup of pasta water, then drain and set aside.

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

Add 8 oz of sliced mushrooms to the skillet. Cook for 6-8 minutes, stirring frequently until tender. Add minced garlic and sauté for an additional minute.

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

Fold the drained pasta into the sauce and add reserved pasta water as needed until reaching the desired creaminess. Stir in the remaining Gruyere cheese and season with salt and pepper.

Plate the pasta immediately and garnish with chopped parsley if desired. Serve hot, optionally with a side salad or garlic bread.