Add the olive oil and butter to a large soup pot over medium heat, followed by the onion and garlic. Cook, stirring occasionally for about 5 minutes. Add the tomatoes and tomato paste, stir, and let them cook for another 5-7 minutes.
Stir in the salt, pepper, and vegetable broth. Bring to a boil then lower heat to medium-low and cook, uncovered, for 10 minutes to steam off some of the tomato juices.
Use an immersion blender to puree the soup, or carefully transfer it in batches to a blender and blend until smooth. Add the tortellini to the soup, increase the heat to medium, and cook for another 7-8 minutes, or until the tortellini is tender but not falling apart.
Stir in the Greek yogurt, taste, and adjust seasonings as desired. Ladle into bowls and enjoy warm, perhaps topped with fresh basil and a side of crunchy bread.
Notes
Possible additions: Red pepper flakes (⅛ to ¼ tsp, add at the same time as the salt)Tomatoes: You can use heirloom, cherry or grape, Roma, or any other variety. Just be sure to use ~8 cups worth.Nutrition: Please note the sodium in your soup may be lower if you choose to use low sodium stock or a lower sodium package of cheese tortellini.Storage: This is best served right away, but can be stored in the fridge for up to 5 days (the tortellini will absorb some moisture during storage and may get soggy). You can also store in the freezer. Thaw in the fridge before reheating on the stove or in the microwave.