Put the fish broth and wine in a saucepan and bring to the boil. Cook down to about ¼ cup. Set aside.
Peel and core the tomatoes. Cut each tomato in half and squeeze to extract and discard most of the seeds and juicy pulp. Chop the tomatoes. There should be about 1¼ cups.
Heat half the butter in a saucepan and add the onion. Cook until wilted.