Season the fish fillets with salt and pepper. Coat in the flour, then dip in the egg and then into the breadcrumbs.
Place on a plate and chill for at least 30 minutes.
Heat the olive oil in a deep heavy-based frypan over medium heat. In 2 batches, add the fish and cook for 3-4 minutes each side until golden and cooked through. Drain on paper towel.
Meanwhile, for tartare sauce, combine ingredients in a bowl and season to taste.
Spread a little tartare onto bun bases, then top with watercress, tomato, avocado and fish. Dollop with a little more tartare and place bun lids on top. Serve with lemon.