Combine olive oil and butter in a large soup pot or Dutch oven; set it over medium heat and cook until the butter is melted.
Add onions; cook for 3 to 4 minutes or until translucent. Add garlic and continue to cook for 20 seconds or until fragrant.
Increase heat to medium-high and stir in the canned diced tomatoes and the tomato sauce; bring to a boil.
Add vegetable broth and bring to a boil. Stir in the frozen tortellini and baby spinach.
Season with salt and pepper and 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 salt and pepper and adjust accordingly.
Ladle into soup bowls and serve.
Notes
Try different tortellini fillings like cheese or chicken tortellini, ricotta-spinach, or even ravioli.
For a creamy twist, mix in 1/2 to 1 cup of heavy cream.
Mix in cooked shredded chicken or ground Italian sausage.
Add more veggies like diced carrots, peas, sautéed mushrooms, and bell peppers.
This soup keeps for 5 days in the fridge or 3 months in the freezer.