Put the tomato on to stew, and the milk in a double kettle to boil, reserving however, half a cupful to mix with flour.
Mix the flour smoothly with this cold milk, stir into the boiling milk, and cook ten minutes.
To the tomato add the soda; stir well, and rub through a strainer that is fine enough to keep back the seeds.
Add butter, salt and pepper to the milk, and then the tomato.
If half the rule is made, stir the tomato well in the can before dividing, as the liquid portion is the more acid.