Heat the oil in a large, heavy, skillet. Add the onion and cook over low heat until it is tender and golden, about 10 minutes. Stir in the garlic, cook another minute, then add the zucchini.
Cook, stirring from time to time, until the zucchini is tender, about 10 minutes longer.
Season with salt and pepper and stir in the butter.
To serve with fish, stir in the mint. Or serve as a vegetable topped with freshly grated Parmesan cheese.