Preheat the oven to 200ºC, fan 180ºC, gas 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 following pack instructions. Mix together the mayonnaise, capers, gherkin and lemon zest. 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. Serve with the sweet potato chips.