Slice the zucchini into ¼-inch-thick slices. You can leave them unpeeled.
Preheat your broiler on high and position an oven rack 4 inches below the heating element (in most ovens this is simply the top rack). Line a rimmed baking sheet with foil and spray it with olive oil.
Arrange the zucchini slices on the prepared baking sheet. Lightly spray them with olive oil and sprinkle them with salt, pepper and garlic powder.
Broil the zucchini until tender and lightly browned, about 2 minutes per side. Keep a close eye on them! Some might brown faster than the others – remove them to a plate and keep broiling the rest.
Remove the pan from the oven. Spoon a little marinara sauce on top of each zucchini slice. Top with cheese and pepperoni. Return to the broiler just until cheese is melted, 30-60 more seconds. Serve immediately.