Begin by bringing a large pot of salted water to a rolling boil over high heat.

Add 12 ounces of fettuccine or linguine and cook according to package instructions, typically about 8–10 minutes, until al dente.

Once cooked, reserve ½ cup of the pasta water for later use, then drain the pasta and set it aside.

In a large skillet, melt 2 tablespoons of unsalted butter and add 2 tablespoons of olive oil over medium heat.

Add the sliced leeks, cooking them for 12–15 minutes until golden brown.

With the leeks caramelized, add 8 ounces of sliced mushrooms and sauté for 6–8 minutes.

Stir in 2 minced garlic cloves and cook for an additional minute until aromatic.

Reduce heat to low, add ½ cup of heavy cream, and stir in ¾ cup of shredded Gruyere cheese.

Stir until smooth and silky, ensuring it's warmed through without bubbling.

Gently fold the cooked pasta into the skillet with the cheese sauce, adding reserved pasta water if needed.

Finish with remaining Gruyere cheese and seasoning with salt and pepper.

Serve immediately on plates and garnish with chopped fresh parsley if desired.