Toss tomatoes, onions, garlic, olive oil, salt, and pepper in a large bowl. Transfer mixture to a baking sheet and roast in the preheated oven for 30 minutes, or until tomatoes are very soft. Set aside.
Heat a large pot on medium high heat and put in roasted tomatoes mixture. Stir in the broth, sugar, red pepper flakes. Bring to a boil, reduce the heat, and simmer about 10 minutes. Puree the soup, in a blender or with hand blender, until smooth.