Bring a large pot of salted water to a boil. Cook pasta according to package directions until al dente. Reserve 1 cup of pasta water, then drain and set aside.
Heat a large skillet over medium-high heat. Add the sausage, breaking it into pieces, and cook until browned and cooked through, about 6–8 minutes. Transfer to paper towels to drain.
Reduce heat to medium, melt butter in the skillet, and sauté garlic for 30 seconds. Pour in heavy cream and bring to a gentle simmer. Whisk in Parmesan in small handfuls until smooth. Season with salt, pepper, and nutmeg. Thin with reserved pasta water if necessary.
Return sausage to the skillet, add the drained pasta, and toss to coat in the sauce over low heat. Adjust consistency with pasta water. Garnish with parsley, extra Parmesan, and red pepper flakes.
Notes
Use freshly grated Parmesan for the best texture.
Swap half-and-half for heavy cream to cut calories, adding a bit of cream cheese to thicken if needed.
Add spinach or peas at the end for a quick vegetable boost.