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

In a large skillet, heat 2 tablespoons of unsalted butter and 2 tablespoons of olive oil over medium heat. Add 2 large sliced leeks to the skillet and sauté for about 12-15 minutes, stirring occasionally, until the leeks turn golden and caramelized.

Add 8 oz of sliced cremini or button mushrooms to the skillet with the caramelized leeks. Cook for 6-8 minutes, stirring occasionally, until the mushrooms are tender and start to brown. Stir in 2 minced garlic cloves and sauté for an additional minute.

Reduce the heat to low and pour in ½ cup of heavy cream into the skillet. Add ¾ cup of shredded Gruyere cheese, stirring gently until the cheese melts and the sauce becomes creamy, about 3-5 minutes.

Once the sauce is ready, add the drained pasta directly into the skillet, tossing everything together. If the sauce seems thick, gradually mix in the reserved pasta water until you reach your desired consistency.

Serve the Mushroom Gruyere Pasta warm in individual bowls or a large serving dish. Garnish with freshly chopped parsley, if desired.