Melt butter in a large Dutch oven over medium heat. Add onion and garlic and cook until softened, about 5 minutes.
Add chicken broth and Alfredo sauce to pot and heat through. Stir in sour cream and cooked chicken.
Bring to a simmer and add tortellini. Cook for 10-15 minutes, or until tortellini is almost done. Add chopped spinach and cook a few minutes longer, until spinach is just wilted.
Season with salt and pepper to taste. Garnish withe grated Parmesan cheese, if desired.
