In an 8- or 9-inch square pan, combine 1 cup coarsely chopped firm-ripe tomatoes, 1 cup coarsely chopped red bell peppers, 2 cloves chopped garlic, and 1 teaspoon olive oil. Cover tightly with foil.
Bake in a 350° oven until tomatoes mash easily, 50 to 60 minutes. Pour vegetable mixture into a blender or food processor. Add 1/2 cup fresh cilantro, 1/3 cup chopped green onions, 1 1/2 teaspoons balsamic vinegar, and 1/4 teaspoon sugar. Whirl until coarsely puréed. Add salt to taste.