Bring a large pot of salted water to a boil and cook the refrigerated cheese tortellini according to package directions; drain and set aside.
While the pasta cooks, melt the butter in a medium saucepan over medium heat.
Whisk in the flour and cook for about 1 minute to form a roux.
Slowly whisk in the chicken broth until smooth and lump-free, about 30 seconds.
Add the heavy whipping cream and garlic powder, whisking until the sauce is smooth; simmer for 2–3 minutes and thin with a little more broth if needed.
Stir in the grated Parmesan and a pinch of nutmeg until the cheese is melted and the sauce is well combined.
Add the cooked chicken and drained tortellini to the sauce, tossing gently to coat; season with salt and pepper to taste.
Serve immediately, garnished with fresh herbs if desired.