Cut onion half into quarters.
Heat a skillet over medium-high heat. Add onion, chiles, tomatoes, and garlic; cook 5 minutes or until blackened, turning frequently. Cool.
Trim root from onion wedges; discard root. Core tomatoes. Finely mince onion, chiles, and tomatoes together until mixture is almost pastelike. Sprinkle with salt; stir well.