Place one onion chopped up very fine in a saucepan with half a cupful of vinegar ; reduce until almost dry, and then add one pint of Espagnole sauce (No. 151), one tablespoonful of capers, three small gherkins and three mushrooms, all finely chopped up together. Cook for ten minutes; season with the third of a tablespoonful of salt, and a scant teaspoonful of pepper, and serve.