In a small basin, combine the biscuit crumbs with the Grape-Nuts and brown sugar, then pour in the melted butter and mix thoroughly.
Sprinkle a third of this mixture over the base of the loaf tin. Now put the egg yolks, grated zests and strained lemon and lime juice into a bowl, and add the caster sugar and gelatine, too. Then place the bowl over a pan of barely simmering water and whisk using an electric hand whisk for about 10 minutes or until the mixture thickens. Then remove the bowl from the heat and whisk again for a further 7-8 minutes until the mixture cools. Now clean and dry the whisk thoroughly and first beat the egg whites till stiff, then beat the cream until floppy but not too thick. You can watch how to separate eggs and whisk egg whites in our Cookery School Video on the right.
Next, take a metal spoon and carefully fold the egg whites first and then the cream into the lemon-and-lime mixture. Now pour it into the tin, sprinkle the remaining crumbs over the top, cover with kitchen foil and transfer it to the freezer and freeze overnight. Three hours before serving transfer the tin to the main body of the fridge, then turn it out on to a serving dish and serve cut into slices. If you have any left over, there is no need to re-freeze it; just leave it in the fridge where it will become more like a mousse.
Note: this recipe contains raw eggs.
You will also need a Delia Online Loaf Tin (or standard 2lb loaf tin), brushed with a tasteless oil such as groundnut oil.