Start by bringing a large pot of salted water to a rolling boil. Add the fettuccine and cook according to the package instructions until al dente. The starchy water from the pasta will be used later to adjust the sauce consistency, so don't forget to reserve a ladleful before draining.

In a large pan over medium heat, melt the butter until it begins to foam. Add the chopped onion and sauté for about 2 minutes until it starts to turn translucent.

Add the sliced mushrooms to the pan and cook for 4-5 minutes, allowing them to release their moisture and develop a golden-brown color.

Once the mushrooms are browned, add the minced garlic and thyme. Sauté for another minute. Season with salt and pepper to taste. Now, add the Truffle Boursin cheese to the pan with a ladle of the reserved pasta water.

Stir gently and let the heat melt the cheese into a creamy sauce. If the sauce is too thick, add more pasta water a little at a time until you reach your desired consistency.

Stir in the chopped parsley and, if using, sprinkle with freshly grated Parmesan. This final touch adds a nutty richness that complements the truffle flavor.

Drain the cooked fettuccine and add it directly to the pan with the sauce. Toss the pasta gently in the sauce until every strand is thoroughly coated.

Serve the Truffle Boursin Pasta immediately, garnished with additional parsley.