Clean the mushrooms. Add the garlic and mushrooms to a large skillet with 2 tablespoons of the butter. Sauté until the mushrooms are soft with deep brown color, about 10-15 minutes. Add the cream and the rest of the butter. Simmer over low heat.

Cook the fettuccine in a large pot according to package directions. Drain, reserving a little bit of the water, and return to the pan.

Add the mushroom sauce to the hot fettuccine in the pan. Toss with tongs to mix. Add Parmesan and up to 1 cup of reserved pasta water as needed to get the consistency right. Season with salt and pepper.