Cook the potatoes in lightly salted boiling water for about 20 minutes or until soft.
Meanwhile, mix together the ingredients for the crust and tip the mixture on to a plate. Put the chicken breasts between 2 sheets of clingfilm or greaseproof paper and batter with a rolling pin or mallet until flat. Dip the chicken into the beaten egg, then press into the crust mixture to coat.
Heat the oil in a large frying pan and cook the chicken for 3 minutes on each side or until cooked through and golden. Set aside and keep warm.
Add half the butter to the pan and stir in the tomatoes, capers and white wine. Season and allow to bubble for 2–3 minutes.
Drain and mash the potatoes with the lemon juice, the remaining butter and plenty of seasoning. Spoon on to serving plates with the crispy chicken. Quickly stir the rocket into the tomatoes and pile a little on to each piece of chicken. Serve immediately.