Heat the oil in a Dutch oven or soup pot over medium-high heat. Add the onions, garlic, Italian seasoning, pepper flakes, salt and pepper; sauté for 4 minutes, or until the onions are tender.
Stir in the broth and tomatoes. Bring mixture to a boil; add tortellini. Reduce heat and gently boil stirring occasionally, for about 7 minutes, or until the tortellini are cooked. Stir in the spinach; cook for 30 seconds or until wilted.