If using fresh tomatoes, peel and chop them.

In a large pot, melt butter over medium heat. Add onion and cook until softened.

Add tomatoes, vegetable broth, and sugar. Bring to a boil, then reduce heat and simmer for 15-20 minutes.

If desired, mix cornstarch with 2 tablespoons of cold water to form a slurry. Stir into the soup and cook for 1-2 minutes, or until thickened.

Season with salt and pepper to taste.