Soak the tripe for several hours, then scrape it thoroughly clean, put it in salted water, and simmer it for three or four hours, until it is like jelly. Drain off the water, and put the tripe aside until ready to use. Put the butter in a saucepan; when hot add the flour, and cook for a few minutes, but do not brown. Then add slowly the milk, and stir until smooth. Add the salt, pepper, and onion juice; then add one cupful of the boiled tripe. Stir until the tripe is heated, and serve immediately.