Spear the stem end of the tomatoes, one at a time, with a two-pronged fork. Dip into boiling water for about 9 seconds. Remove and pull away the peel with a paring knife. Pare away the stem end. Chop the tomatoes coarsely.
Melt the butter in a saucepan and add the onion. Cook until onion is wilted and add the tomatoes and remaining ingredients. Simmer about 5 minutes. If fresh thyme sprigs are used, discard them and discard the bay leaf.