For the soufflés, preheat the oven to 180C/160C Fan/Gas 4.
Rub the inside of four ramekins with butter, then dust with flour and place in a high-sided oven tray.
Melt the butter in a large saucepan set over a medium heat, then add the flour and mix well.
Cook over low heat for 2-3 minutes, stirring with a wooden spoon. Gradually whisk in the milk, a little at a time, stirring constantly to avoid any lumps forming. Continue until all the milk has been used, then reduce the heat to very low and cook for a further 2-3 minutes. Add 125g/4½oz of the cheese and mustard, then beat the egg yolks into the mixture and season with salt and pepper.
In a clean, dry bowl, whisk the egg whites until they hold firm peaks when the whisk is removed from the bowl, then fold them into the soufflé mixture.
Divide half the mixture among the soufflé dishes and drop a whole scallop in each, then add the rest of the mix. Pour enough hot water into the oven tray to reach halfway up the sides of the dishes. Bake the soufflés for 15-20 minutes, or until they are risen and evenly browned.
Remove the dishes from the tin and set aside for a few minutes. Run a knife around the edges of the soufflés and turn them out onto individual ovenproof dishes.
Preheat the grill to medium-high. Coat the tops of the soufflés with the cream, then scatter over the remaining cheese and place under the grill until the tops are golden-brown.
For the shellfish sauce, heat a large frying pan, add 15g/½oz of butter, the shallots, carrot and thyme and cook for 2-3 minutes, or until just softened but not coloured.
Add the lobster and langoustine shells, cayenne pepper and tomato purée and cook for one minute.
Add the white wine and flambé the liquid by slowly tilting the pan towards the gas flame to set alight, or lighting it with a match. Let the flames flare up then die down. (CAUTION: keep your face and hair away from the flames. Do not leave unattended.)
Add the chicken stock, bring to the boil, then reduce the heat and simmer for 5-8 minutes. Blend in a food processor to a purée. Press the mixture through a fine sieve into a saucepan, whisk in the cream and season to taste with salt and pepper.
For the clams and mussels, heat a large, lidded saucepan and add the clams, mussels and white wine. Put the lid on and cook for 2-3 minutes, or until all of the clams and mussels have opened. (Discard any clams or mussels that do not open fully.)
For the vegetables, cook the turnips and carrots in a pan of boiling water for 2 minutes, then add the asparagus, and boil for a minute. Drain and pat dry. Heat a frying pan until hot, add the butter and fry the vegetables for 1-2 minutes, or until pale golden-brown.
Heat a frying pan until hot, add the butter and fry the vegetables for 1-2 minutes, or until pale golden-brown.
Heat another frying pan, add 15g/½oz of butter and fry the lobster meat and langoustine meat for about 1-2 minutes on each side.
To serve, place a soufflé on each serving plate. Garnish with the cooked shellfish, baby vegetables, sauce and chervil.