Preheat the slow cooker if necessary; see the manufacturer's instructions. Butter the inside of a soufflé dish that is 14 cm (5½ inches) across and 9 cm (3½ inches) high. Put the flour in a bowl, add the measured butter and rub in with the fingertips until the mixture resembles fine breadcrumbs. Stir in the sugar, then mix in the eggs and milk until smooth. Stir in the apple.
Spoon the mixture into the soufflé dish and spread it level. Sprinkle the sugar for the sauce over the top and dot with the 25 g (1 oz) butter. Pour the measured boiling water over the top, then cover loosely with foil.
Lower the dish carefully into the slow cooker pot using a string handle or foil straps (see
Lift the dish out of the slow cooker. Remove the foil and loosen the sides of the sponge. Cover with a dish that is large enough to catch the sauce, then invert and remove the soufflé dish. Serve with spoonfuls of vanilla ice cream, crème fraîche or pouring cream.