1. Preheat the oven to 180°C/Gas mark 4.
2. Melt the butter in a pan over a medium heat and sauté the onion until soft but not coloured. Add the mustard, horseradish and lemon juice, and then stir in the cream.
3. Once the sauce is combined, stir in the white crab meat.
4. Spoon the crab mixture into the cleaned shells, place on a baking sheet and bake for 8 minutes. Meanwhile, combine the cheese, breadcrumbs and parsley in a bowl with a little salt and pepper. Set aside.
5. To make the crab butter, place the brown crab meat into a food processor with the horseradish cream and blitz slowly adding the cream until a mayonnaise consistency is reached.
6. When the crab is cooked, remove from the oven and cover the crab with the breadcrumb mixture and place under a hot grill for 2-3 minutes, or until golden.
7. Serve with hot granary toast and a small pot of crab butter on the side.