Heat 2 tablespoons of olive oil in a large pot. Sauté onions until softened, 6 to 8 minutes. Add minced garlic and Italian seasoning; stir for 1 minute until fragrant.

Sprinkle the flour and cook for 1 minute, stirring to coat. Gradually add the broth and stir until the flour dissolves.

Add canned diced tomatoes with all its juices. Bring to a boil, then reduce to a simmer for 5 to 6 minutes with the lid partially covered.

Blend the soup using an immersion blender until smooth. If needed, add more broth.

Stir in granulated sugar to reduce acidity. Adjust sweetness with additional sugar if necessary, and season with salt and pepper to taste.

Add the tortellini and simmer for 8 to 10 minutes with the lid half-covered. If using frozen tortellini, cook for an extra couple of minutes.

Stir in heavy cream or half-and-half to taste. If the soup is too thick, add more broth. Then add the spinach and simmer for another 1 to 2 minutes.

Let cool slightly before serving. Top with grated parmesan cheese when serving.