Open the crabs, remove all the white and brown meat and place in a bowl. Feel for any small pieces of broken shell and remove from the meat. Wash the main shells of the crabs thoroughly and pat dry using kitchen paper.
Use a fork to gently mash together the brown and white crabmeat. Season with salt and pepper and spoon the crabmeat back into the clean shells.
To make the rarebit sauce, place the cheese and milk into a small saucepan and heat, stirring occasionally to prevent it catching, until the cheese is melted. Add the breadcrumbs and flour and cook for 2-3 minutes, stirring all the time to prevent any lumps forming.
Remove from the heat and allow to cool. Add the Worcestershire sauce, whole egg, egg yolk and the mustard powder. Mix thoroughly and season, to taste, with salt and pepper. Set aside while you make the fries.
Preheat the deep-fat fryer to 180C. (CAUTION: hot fat can be dangerous. Do not leave unattended.)
Drop the fries into the hot fat and cook for 3 minutes, or until golden-brown and crisp. Drain on kitchen roll and season with salt.
For the crab rarebit, preheat the grill to high.
Place the filled crab shells on a baking tray. Spoon the rarebit sauce over the top of the crabmeat. Drizzle a little melted butter over the top and place under the hot grill for a few minutes until the rarebit is bubbling and golden-brown.
When ready to serve, dress the Little Gem leaves with the olive oil and lemon juice.
Serve the crab rarebit in the shells with the salad and fries alongside.