Blanch the tripe and cut into 3-in. squares. Put in a saucepan with the milk, ½ pt. water and the salt; bring to the boil. Peel and slice the onions finely. Add them to the tripe and simmer very slowly for 2 hr. Mix the flour to a smooth paste with a little cold milk and add to the pan. Stir with a wooden spoon until boiling. Simmer for another 10 min., season to taste and serve.