Lightly brush the grate with oil. Place the tomato, stem side down, in the center of the grill. Cover and cook, turning occasionally, until softened and charred all over, about 10 minutes. Meanwhile, thread the garlic clove and onion onto a skewer. Place the skewer at the edge of the fire, where it is cooler. Cover and cook, turning, until lightly browned and just softened, about 8 minutes.