Heat butter over medium/high heat in a large dutch oven or stock pot. Add in the diced onion and cook for 3-4 minutes, stirring occasionally. Add in the garlic and saute for 30 seconds until the garlic and onion are translucent and fragrant.
Add the chicken broth, pesto, and tomatoes, stirring to combine.
Bring the soup to a boil and then reduce heat to low, stirring in the tortellini.
Season with salt and pepper to taste.
Set to simmer and cook for approximately 7 minutes or until the tortellini is al dente.
Stir in the cooked chicken, spinach, and white beans.
Stir to combine and allow to simmer until the spinach is wilted to your liking.