Scrape the skins of the squash. Cut them in four, lengthwise, then in four the other way. Wash and drain them. Cut a tomato in four. Peel, seed and chop the other two. Cut the onion in half.

Melt the butter in a saucepan. Put the four tomato pieces on the bottom (skin down), then add the pieces of squash, and over this the chopped tomatoes, salt, chopped mint and onion. Boil, then simmer for 45 minutes.