Preheat the oven to 190C/ 180C fan/gas mark 5. Melt the butter in a medium saucepan and add the onion. Sauté over a medium-low heat until the onion is soft but not coloured, about 10 minutes. Add the cayenne and flour and stir everything around.
Take the pan off the heat and slowly add the milk and cider a little at a time, stirring constantly. Once it’s all been added put the pan back over a medium-low heat, season, and bring the mixture up to the boil, again stirring constantly.
Add the cream, and bring to a simmer. Take off the heat and add the mustard, nutmeg and lemon juice.
Spread the brown crab in the bottom of a gratin dish (or you can use individual gratin dishes or ramekins). Put the white meat on top. Pour over the cream mixture and sprinkle on the cheddar, breadcrumbs and parsley. Put into the oven and bake for 15-20 minutes. The top should be golden.