This is a recipe that did not quite work. The fault was in part mine; but also in part -- I hesitate to say it -- that of the recipe, which comes from my favourite cookbook, Richard Olney's Simple French Food (available in hardback from Grub Street).
500 g apples, quartered, cored, peeled, sliced
Large handful stale breadcrumbs, without crusts
About 125 g butter
250 ml milk
75 g sugar
Small pinch salt
Cook the apples in 60 g butter, tossing from time to time, until tender. Cook the breadcrumbs in the remaining butter over very low heat, stirring regularly, until they are golden and crisp.
Spread the crumbs in the bottom of a lightly buttered gratin dish, arrange the apples on top, and sprinkle lightly with cinnamon. Whisk together the milk, eggs, sugar, and salt, and pour the liquid over the apples. Bake at gas mark 3/160 C (3 is the equivalent of 170 C in Olney's book) for about 30 minutes.
Olney advises that you use russets. My mistake was to use bramleys, which you cannot saute as he instructs, because they break down. So they turned the breadcrumbs soggy. But if they had not, the custard would have.
What you want here is three, distinct layers: crisp breadcrumbs, tart apple segments, fluffy topping. Perhaps a topping of whisked egg whites, forming a meringue, would work best.