1. Sauté onion for a few minutes. Lower heat, add garlic, and sauté until fragrant (about three to four minutes).
2. Add salt, pepper and oregano. Add diced tomatoes. Bring up to a simmer, then add vegetable broth. Bring to a boil.
3. Add frozen tortellini and simmer nine minutes, stirring occasionally. Add spinach and use a spoon or spatula to submerge in the hot soup to assist in wilting. Simmer one minute more and serve.