Place the whole live scallops onto a hotplate or hot griddle pan for 1-2 minutes, until they begin to open.
Slide the blade of a filleting knife between the flat shell and the scallop and carefully lift off the shell lid.
Slide the knife blade between the scallop and the concave shell to detach the scallop and coral.
Keep the four best concave shell halves for serving, brush them under cold water and dry them out.
Peel off the membrane surrounding the scallops, clean them in cold water, cut in pieces and set aside for use in the sauce.
Separate the whites of the scallops from the orange corals and wash gently in cold water to remove any grains of sand.
Place the scallops, corals, mushrooms, membranes and lemon juice into a pan with the fish stock, set over low heat and slowly bring to the boil. Lower the heat to simmer very gently for one minute. Remove from the heat, set aside and keep the scallops and mushrooms in the stock at room temperature.
Preheat the oven to 220C/425F/Gas 7.
Scrub the potatoes under cold running water and prick in 2-3 places with the point of a knife. Place onto a baking sheet and transfer to the oven to bake in their jackets for 35-40 minutes, or until soft inside.
Halve the potatoes and scoop out the cooked flesh from the jackets. Force the potato flesh through a hard sieve or ricer into a clean bowl.
Add two of the egg yolks and the butter and beat until smooth. Season, to taste, with salt and freshly ground white pepper. (The potato can be done the day before and kept in the fridge until needed.)
For the sauce, melt the butter in a saucepan, stir in the flour and cook for two minutes to make a roux.
Take the pan off the heat and strain in the stock from the scallops and mushrooms, stirring constantly to combine without lumps. Set the scallops and mushrooms aside.
Simmer the sauce gently for a few minutes, whisking frequently.
In a bowl, mix the two egg yolks with the cream, and then pour this mixture into the sauce. Before it begins to bubble, take the pan off the heat, season the sauce with salt and freshly ground black pepper and keep hot.
For the tabbouleh, place the couscous into a bowl and pour an equal amount of boiling water onto it, season with salt and freshly ground black pepper, stir with a fork and cover with cling film for 4-5 minutes. Stir again and add the olive oil.
Shave the very top of the broccoli florets off with a sharp knife and add to the couscous. Stir gently with a fork and season, to taste, with salt and freshly ground black pepper.
To assemble the final dish, increase the oven temperature to 240C/465F/Gas 9.
Cut the scallop whites into thick slices. Place a layer of mushrooms into each of the four clean scallop shells. Add a layer of scallop whites then top with the scallop corals.
Fill a piping bag with the potato mixture and pipe an attractive ribbon around the edge of the scallop shells.
Mix the gruyère and breadcrumbs together in a clean bowl.
Cover the mushroom and scallop layers generously with sauce and sprinkle the top with the gruyère and breadcrumbs mixture.
Melt the butter and pour it over the top of the sauce.
Place the scallops into the hot oven to bake for 3-5 minutes, until the top of the sauce is lightly glazed and the potato border pale golden.
To serve, spoon the tabbouleh out onto four plates and carefully remove the scallops from the oven and place one on top of each pile of tabbouleh. Do not forget to warn your guests that the scallop shell will be extremely hot.