1. Crush the potato chips into large crumbs and place them in a shallow plate. Do the same with the beaten eggs and flour.
2. Cut the fish into portions and roll them in the flour. Tap them to remove the excess, dip them in the beaten eggs, and then coat them with the potato chips. (If you have time, cover and let them rest in the fridge for 30 minutes to set the breading.)
3. Heat the oil in a large skillet and brown the fish for 5 minutes on each side over medium heat.
4. Put the pickles and small onions in a food processor and pulse a few times to chop them, but not into crumbs. Pour into a bowl, add the mayonnaise and some chopped watercress, as well as the capers. Drain the fish on paper towels.
5. Serve with the sauce, the remaining watercress, and a lemon wedge. Complete with mashed potatoes, potato wedges, or your preferred side dish.