Combine olive oil and butter in a large soup pot; place over medium heat and cook until butter is melted.
Add onions; cook for 2 minutes, or until translucent.
Add garlic and continue to cook for 2 minutes, stirring occasionally.
Set heat to medium-high and stir in diced tomatoes and tomato sauce; bring to a boil.
Add vegetable broth; bring to a boil.
Add frozen tortellini and spinach.
Season with salt and fresh ground pepper.
Continue to cook over medium-high heat, stirring occasionally, for 10 minutes, or until tortellini is tender and soup is heated through.
Remove from heat and let stand 5 minutes.
Taste for seasoning and adjust accordingly.