Rinse chicken breasts and dab dry with kitchen towels. Cover with cling film and beat lightly with a meat mallet.
Whisk egg and mustard together in a bowl. Add milk, flour, and a little salt. The batter will be thicker than pancake batter, but should still be runny.
Heat a generous amount of oil in a pan on high heat.
Sprinkle the chicken with salt and pepper and dip into the batter. Drain any access and place on the hot oil. Fry uncovered till brown on each side.