Ask the fishmonger to clean out the trout. Holding each one by the tail, roll in seasoned flour to coat all over. Heat the butter and oil in a large frying pan and when hot, but not brown, add the fish one at a time.
Cook over moderate heat for about 5 minutes until nicely browned, then turn and cook the second side for a further 5 minutes. Lift from the pan on to a hot serving planer. Add the juice of the lemon to the hot butter. Heat for a moment, then pour over the fish and serve.