This involves placing the tripe in a large pan of cold water which is slowly heated to the boil, so that the impurities rise to the surface. The butcher may have done this for you, so ask.

Put the tripe into a saucepan with a quarter cupful of water, with the lid on, place on the hob for 8-10 minutes approx. Discard the liquor in the pot, add the sliced onion and cover with cold milk.