Place an oven rack in the center position of the oven. Preheat the oven to 425°F. Line a baking sheet with foil (lightly misted with nonstick cooking spray) OR parchment paper.
Wash and dry the zucchini and squash, and then cut each one into ¼-inch thick slices. Evenly arrange the zucchini and squash rounds on the prepared pan. Wash, dry, and cut each tomato in half and place the tomato halves, cut sides up, in the empty spaces between the zucchini and squash rounds.
Lightly sprinkle the vegetables with garlic salt and freshly ground black pepper. Use a small spoon to spread a thin layer of Parmesan cheese on each veggie.
Bake for 15 to 20 minutes, or until the Parmesan melts and turns a light golden brown. (Watch these closely the first time you make them and pull them out of the oven early if the Parmesan is golden before 15 minutes. Alternatively, you may broil them for a minute or two at the end of the cooking time to speed up the browning.) Serve immediately.