Heat the olive oil in a large saucepan. Add the garlic and onion and cook over moderate heat, stirring, until softened, about 4 minutes. Add the zucchini, saffron, cumin and turmeric and season with salt and pepper. Cook, stirring occasionally, until barely tender, about 8 minutes. Transfer to a bowl and refrigerate until chilled, about 20 minutes. Stir in the lemon juice, season with salt and pepper and sprinkle with the mint. Serve chilled or at room temperature.