Combine the relish, jalapeños and mayonnaise to make the tartar sauce and refrigerate (can be made up to 48 hours in advance.)
Rinse the fish fillets and pat dry.
Blend the flour, cayenne pepper and salt, coat the fillets and shake off the excess.
Heat the butter over moderate heat and pan fry the fish about 2 minutes on each side or until the flesh can be flaked with a fork. Serve the fish with the tartar sauce and garnish with wedges of lime or lemon.