Cook 16 ounces dry pasta according to package directions until just al dente; drain.
While pasta cooks, melt 3 tablespoons butter in a large skillet over medium heat. Add 2-3 cloves garlic and cook until fragrant, 1-2 minutes. Stir in 3 tablespoons flour.
Whisk in 1 cup chicken broth until smooth, then stir in 2 cups half and half.
Add 1/2 teaspoon salt, 1/4 teaspoon pepper, 1/4 teaspoon cayenne pepper, and 1/4 teaspoon red pepper flakes and simmer until sauce is thickened, about 5 minutes.
Stir in 1/2 cup grated Parmesan cheese and 1 cup mozzarella cheese, until cheeses are melted and smooth.
Slice 1 12 oz fully cooked smoked sausage, any variety into 1/4 inch pieces and add to the sauce, along with the chopped 1-2 tablespoons chopped fresh parsley and cooked pasta. Season with additional salt and pepper if needed.
Preheat broiler. Pour the pasta mixture into a lightly greased, 9x13 inch baking dish. Top with remaining 1 cup mozzarella cheese.
Broil for 2-3 minutes, or until cheese is bubbly and golden.