Drizzle peaches and onion with oil and rub to coat. On a grill or in a grill pan over high heat, grill peaches and onion until peaches are lightly charred and skins begin to loosen, about 10 minutes, rotating occasionally. Let cool.
Remove and discard peach skins. Chop peaches into medium chunks and onion into smaller chunks. Combine in a bowl with remaining ingredients and season with salt and pepper.
To store, refrigerate, up to 1 week.