Preheat the oven to 200ºC/fan 180ºC/gas mark 6. Cut the sweet potatoes into chunky chips. Place in a roasting pan with the oil and toss together. Cook for 30-35 minutes, turning halfway.
Meanwhile, grill the fish fingers according to the instructions on the packet. Mix together the mayonnaise, capers, gherkin and lemon zest and season to taste.
Warm the ciabatta in the oven for a couple of minutes. Halve, then split each half. Fill with the homemade tartare sauce, fish fingers, salad leaves and tomato and serve with the sweet potato chips.