Brush or spray the remaining onion slices with oil and lay in a single layer in the center (the least hot part) of the
grill, along with the tomatoes. Set the chiles over the hottest part. Roast, turning everything occasionally, until the chiles' skin (but not the flesh) is blistered and uniformly blackened all over, about 5 minutes, and the onion and tomatoes are softened and browned in spots, 10 to 15 minutes, depending on their size and the heat. When the chiles are done, remove them and cover with a kitchen towel. Set the tomatoes aside on a plate. Finely chop the onion and
scoop it into a bowl.