Preheat the oven 400 degrees F. Line a baking sheet with aluminum foil and spray with cooking spray.
Slice the zucchini 1/4-inch thick; you should get about 24 slices. Place the slices on the prepared baking sheet and sprinkle with salt. Top each slice with 1/2 teaspoon marinara sauce and 1 teaspoon mozzarella.
Bake until the cheese is melted and bubbly, about 8 minutes.