1. Heat a 12-inch cast-iron skillet or griddle over medium heat 5 minutes. Place tomatoes, cut side down, in skillet, spacing evenly; add garlic and jalapeño pepper. Cook, turning occasionally, about 6 minutes or until slightly charred and softened. Transfer tomatoes and jalapeño pepper to a blender or food processor. Peel garlic, and place in blender. Add onion wedges to skillet, and cook 5 to 6 minutes or until slightly charred and softened. Transfer onions to blender.
2. Process vegetables 30 to 40 seconds or to desired consistency. Add lime juice and salt to blender, and process until combined. Cool completely (about 10 minutes). Stir in cilantro. Serve at room temperature, or refrigerate in an airtight container up to 3 days.
Note: For a redder salsa, use red jalapeño peppers.