Preheat the slow cooker if necessary; see the manufacturer's instructions. Lightly butter a soufflé dish that is 14 cm (5½ inches) across the base and 9 cm (3½ inches) high, and base-line with a circle of nonstick baking paper.
Cream together the measured butter and sugar in a bowl with a wooden spoon or electric hand mixer. Gradually mix in alternate spoonfuls of beaten egg and flour, and continue adding and beating until the mixture is smooth. Stir in the poppy seeds and lemon rind, then spoon the mixture into the soufflé dish and spread the top level. Cover the top of the dish loosely with buttered foil and then lower into the slow cooker pot using foil straps (see
Pour boiling water into the slow cooker pot so that it comes halfway up the sides of the dish. Cover with the lid and cook on high for 4½–5 hours or until the cake is dry and springs back when pressed with a fingertip.
Lift the dish carefully out of the slow cooker, remove the foil and loosen the edge of the cake with a knife. Turn out on to a plate or shallow dish with a rim. Quickly warm the lemon juice and sugar together for the syrup and as soon as the sugar has dissolved, pour the syrup over the cake. Leave to cool and for the syrup to soak in. Cut into slices and serve with spoonfuls of crème fraîche, decorated with lemon rind curls.