Preheat oil in a large, deep skillet over medium heat. Add onions and garlic and cook, stirring occasionally, until onions are softened, about five minutes.
Place onion and garlic in blender with tomatoes, cumin, chipotle peppers and adobo sauce; process until smooth.
Return the sauce to the skillet and add meatballs. Bring to a rapid simmer over medium-high heat. Cover, reduce heat to low, and cook 30 minutes, stirring occasionally.
Serve immediately, drizzled with sour cream, if desired.