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, cook the fish-less fillets according to the instructions on the packet. Mix together the vegan 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, fillets, salad leaves and tomato and serve with the sweet potato chips.