Steam the zucchini until soft, 15 to 20 minutes. Drain and, using a fork, mash the zucchini in the colander to squeeze out as much liquid as possible.
Heat 2 tablespoons of the olive oil. Add the onions and cook over moderate heat, stirring, until lightly browned, about 8 minutes. Add the garlic and cook, stirring, until it just begins to color, about 30 seconds. Add the zucchini and mint, season with salt and pepper and cook, stirring, until well mixed and heated through, about 5 minutes.
Stir in the remaining tablespoon of olive oil and serve warm or at room temperature with the lemon wedges.