Butter a 1.5-1.7 litre/2½-3 pint ovenproof dish. Peel and core the apples and slice them thinly. Arrange the slices on the bottom of the dish and sprinkle with the cinnamon and muscovado sugar, then dot with the butter. Pour the lemon juice over the apples.
Preheat the oven to 190°C/375°F/gas mark 5. Put the breadcrumbs into a mixing bowl with the caster sugar, soured cream and lemon rind. Separate the eggs, putting the yolks into the bowl with the breadcrumbs and the whites in another bowl. Stir the breadcrumb mixture well to mix. Add the cream of tartar to the egg whites and whisk until they stand in soft peaks. Using a metal spoon, fold the whisked whites lightly into the breadcrumb mixture. Pour on top of the apples and cook in the centre of the oven for about 40 minutes, until well risen and brown.