01 - Bring a large pot of salted water to a boil. Cook the fettuccine until al dente according to package instructions. Reserve 1/2 cup of the pasta cooking water, then drain.
02 - Meanwhile, heat 2 tablespoons butter in a large skillet over medium heat. Add sliced mushrooms and sauté for 5 to 7 minutes until golden and tender. Remove mushrooms and set aside.
03 - In the same skillet, melt 2 tablespoons butter. Add minced garlic and sauté for 30 seconds until aromatic.
04 - Pour in heavy cream and milk, bringing to a gentle simmer over low heat while stirring occasionally.
05 - Gradually stir in grated Parmesan until fully melted and smooth. Season with salt, freshly ground black pepper, and nutmeg if desired.
06 - Return sautéed mushrooms to the skillet. Add drained fettuccine and toss to combine, adding reserved pasta water as needed to achieve a silky consistency.
07 - Remove from heat and sprinkle with chopped parsley. Serve immediately with extra Parmesan on the side.