Bring a large pot of salted water to a boil and cook the 18 ounces refrigerated cheese tortellini according to package directions until al dente. 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 for about 30 seconds to form a roux, stirring constantly.
Slowly whisk in 1/2 cup chicken broth until smooth and lump-free, about 30 seconds.
Add 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 becomes too thick, thin with 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, the drained tortellini, and fresh herbs if using; toss gently to coat. Season with salt and pepper to taste and serve immediately.