Preheat the oven to 400°F. Brush 1 tablespoon of the oil all over the bottom of a large rimmed baking sheet. Toss the zucchini, onion, and garlic with another 2 tablespoons of the oil and season generously with salt and pepper. Scatter most of the zucchini mixture on the bottom of the baking sheet, then arrange most of the tomatoes on top of the zucchini. Arrange the fish on top of the vegetables, then drizzle 2 tablespoons of olive oil over the fish. Scatter the remaining zucchini mixture and tomatoes on top of the fish. Cut half the lemon into thin slices and arrange them all over the top of the fish and vegetables. Drizzle 2 more tablespoons olive oil on top, then scatter most of the cilantro all over the top.

Bake until the fish is cooked through and the vegetables soften, 13 to 15 minutes. Squeeze the remaining lemon half all over the fish, drizzle with the remaining tablespoon of oil, season with more salt and pepper, and garnish with cilantro.