Bring a large pot of salted water to a boil. Add the fettuccine and cook until al dente, about 8 minutes.
Meanwhile, heat a large skillet over medium-high heat. Add the sausage and cook, breaking it up with a spoon, until it is no longer pink, about 5 minutes.
Add the garlic and cook until fragrant, about 1 minute.
Remove the sausage from the pan and set aside.
In the same skillet, melt the butter over medium heat. Whisk in the flour until no lumps remain. Slowly whisk in the milk until smooth.
Stir in the salt, pepper, and nutmeg. Bring the mixture to a simmer and cook until thickened, about 3 minutes.
Remove from the heat and stir in the Parmesan cheese. Add the cooked sausage back to the pan and stir to combine.
Drain the cooked fettuccine and add it to the skillet. Stir to combine.
Pour the mixture into a 9x13 inch baking dish and sprinkle with the mozzarella cheese.
Bake in a preheated 375°F oven until the cheese is melted and bubbly, about 20 minutes. Serve hot.
Interesting Facts
Alfredo sauce is named after the Italian chef Alfredo di Lelio.
Fettuccine Alfredo is a popular Italian dish, but it's not a traditional Italian recipe.
Sausage is a popular addition to Fettuccine Alfredo.