Dredge fillets in flour and shake to remove excess four. Fry in pan with a little oil until browned on both sides, and the fillets are cooked through.

Transfer fillets to a platter as they are cooked.

Remove all excess oil from pan. Add in the 3 tablespoons butter, and the onions. Cook until the onions and butter are lightly browned. Add in the parsley. Add the cayenne pepper and salt, to taste. Whisk in the lemon juice and the lemon slices. Cook for 2 minutes, then pour over the top of the sanddabs.

Serve hot.