Heat olive oil and butter in a large pot over medium heat. Once the butter melts, add the chopped onion and cook until softened, about 5-7 minutes. Stir in the minced garlic, letting it release its aroma for a minute.
Sprinkle in the flour and stir constantly for another minute. Gradually whisk in the chicken broth, ensuring there are no lumps. This creates a thick, velvety base for your soup.
Add the diced tomatoes, crushed tomatoes, Italian seasoning, and brown sugar to the pot. Stir the mixture and bring it to a gentle boil. Season with salt and pepper to bring out the flavors.
Add the heavy cream and stir well to incorporate. This step gives the soup a rich, indulgent texture.
Reduce the heat to a gentle simmer. Add the cheese tortellini and cook for 7-9 minutes or until tender. Stir occasionally, ensuring the pasta absorbs the flavors of the soup.
Tear fresh basil leaves and add them just before serving. This gives the soup a burst of freshness and a fragrant touch.