Put fish fillets and flour into a bag and shake until coated. Crack in the egg and shake again.
Sprinkle the lemon and herb crumb onto a plate then season with man grind. Coat fish with crumb mixture on both sides.
Heat oil and butter in a large frying pan then cook fish on both sides until golden and fish is cooked through.
To make the tartare, mix together mayo, capers, gherkins, spring onion and the juice of half the lemon.
Cut buns in half and grill or toast until warm. Spread tartare on base of bun and top with lettuce, fish and a squeeze of lemon then the top of the bun.