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

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

Increase the heat slightly and add 8 oz of sliced mushrooms. Cook for 6-8 minutes until tender, then add 2 minced garlic cloves and cook for an additional minute until fragrant.

Lower the heat to medium-low and pour in ½ cup of heavy cream. Gradually add ¾ cup of shredded Gruyere cheese, stirring until it melts into a creamy sauce.

Add the drained pasta to the skillet, tossing gently to coat. If too thick, add reserved pasta water gradually until desired consistency is reached. Mix in additional ¾ cup of Gruyere cheese and season with salt and pepper.

Serve immediately, optionally garnished with chopped parsley for added color and freshness.