Add the zucchini, onion, garlic, chopped tomatoes, 1 tablespoon of oregano, ¼ cup of oil, ½ tablespoon of salt, and 1 teaspoon of pepper.
Toss well, then distribute evenly in a 9-by-13-inch broiler-safe baking pan.
Lay the tomato slices on top, spacing them evenly.
Drizzle with the remaining 2 teaspoons of oil, then sprinkle with the remaining ½ teaspoon oregano and ¼ teaspoon each of salt and pepper.
Bake until a skewer inserted into the vegetables meets no resistance, about 25 minutes.