Slice the tomatoes in half and place them skin side down on the baking sheets. Space them a couple inches apart. Slice the eggplant into 1″ rounds, then slice each round into 1″ cubes. Scatter the eggplant cubes across the baking sheets, in between the tomatoes. Peel the garlic cloves and then crush with the flat side of a knife, leaving them whole. Chop the red onion into 1/2″ chunks. Scatter the garlic and onion in between the tomatoes as well.
*If your baking sheets are quite large and you can fit everything on to one, make sure the vegetables aren’t too crowded.