Bring a large pot of salted water to a boil and cook the refrigerated tortellini according to package directions; drain and set aside.
While the pasta cooks, melt the butter in a medium saucepan over medium heat.
Whisk the flour into the melted butter and cook, whisking, for about 30 seconds to form a roux.
Slowly whisk in the chicken broth until smooth and lump-free, then whisk in the heavy cream and garlic powder.
Simmer the sauce, stirring, for 2–3 minutes until slightly thickened; if it becomes too thick, thin with a little more chicken broth.
Remove the sauce from heat and stir in the grated Parmesan and a pinch of nutmeg until the cheese is melted and the sauce is smooth.
Add the cooked chicken and drained tortellini to the saucepan and toss gently to coat; season with salt and pepper to taste.
Serve immediately, topped with fresh herbs if desired.
