Line a rimmed baking sheet with aluminum foil, adjust an oven rack to the upper-middle position, and preheat the broiler to high.
Trim the ends from the zucchini and slice each zucchini crosswise into 1/2- to 3/4-inch rounds (you should get about 24 rounds). Heat the olive oil in a large nonstick skillet set over medium-high heat. Add the zucchini and cook until browned on one side (in batches, if needed), 4 to 5 minutes.
Transfer the zucchini to the baking sheet, browned side up. Top each zucchini round with 1/2 teaspoon of the marinara sauce, a pepperoni slice (if using), and a piece of mozzarella. Broil the zucchini until the cheese is melted, 2 to 3 minutes (watch the pizza bites closely, as broiler intensities vary).
Remove the zucchini from the oven and transfer to a platter. Sprinkle with the salt and serve warm.
"Supermarket Healthy: Recipes & Know-How For Eating Well Without Spending a Lot" (c) Copyright 2014 by Melissa d'Arabian. Clarkson Potter/Publishers.