Melt the butter in a large soup pot over medium-high heat.
Add the diced onion and minced garlic.
Cook, stirring constantly, until the onion becomes soft and tender.
Pour the chicken broth into the pot with the onion and garlic mixture.
Bring it to a gentle boil.
Add the cheese tortellini to the soup.
Cook for about 7-10 minutes until the tortellini are tender.
Add the spinach to the pot.
Stir until the spinach wilts and combines well with the tortellini.
Season the soup with salt and black pepper.
Serve the soup warm.