Bring a large pot of salted water to a boil and cook the 18 ounces refrigerated cheese tortellini according to package directions; drain and set aside.
While the pasta cooks, melt 4 tablespoons butter in a medium saucepan over medium heat.
Whisk in 2 tablespoons flour and cook, whisking, for about 30 seconds to form a roux.
Slowly whisk in 1/2 cup chicken broth until smooth and lump-free (about 30 seconds).
Whisk in 3/4 cup heavy whipping cream and 1/2 teaspoon garlic powder; continue whisking until the sauce is smooth, then simmer for 2–3 minutes. If the sauce is too thick, add a little more chicken broth.
Stir in 1/2 cup grated Parmesan and a pinch of nutmeg until the cheese melts and the sauce is creamy.
Add 1 1/2 cups cooked chicken and the drained tortellini to the saucepan; toss gently to coat. Season with salt and pepper to taste.
Serve immediately, garnished with fresh herbs if desired.