Reserve 1/2 cup of the milk, put the remainder on to cook in a stew-pan. Mix the flour with the cold milk, and stir into the boiling milk. Cook for 10 minutes, then add the salt, pepper and butter. Stir the soda into the hot tomatoes and stir 1/2 minute, then rub through a strainer. Add the strained tomatoes to the thickened milk, and serve at once.