Preheat broiler to high with oven rack 6 inches from heat. Place tomatoes, onion, and garlic on a broiler pan; broil for 8 minutes or until mostly blackened, turning occasionally. Peel tomatoes; discard peels. Place tomatoes in a blender. Add onion mixture, including juices, to blender. Add chipotle, salt, and sugar; cover with blender lid, and let mixture stand 5 minutes. Process until very smooth, stopping occasionally to scrape down sides.
Heat oil in a medium skillet or saucepan over medium; carefully add the tomato sauce (sauce will splatter). Cook 5 minutes or until salsa thickens, stirring occasionally. Cool and serve.