Cook the lobster in a large pan of boiling water for 3-4 minutes, then remove the lobster from the pot.
Remove the claws and return them to the boiling water for an extra minute.
Heat an ovenproof frying pan, add the lobster, lobster claws and white wine, cover with a circle of greaseproof paper and cook for 1-2 minutes. Turn the lobster and lobster claws over, then cook for further 1-2 minutes.
Remove the lobster from the pan and cut the tail into pieces, with the shell still on. Crack the claws and knuckles and remove the meat. Chop the lobster meat and set aside.
Chop the head and reserved claw shells and return to the pan with the white wine. Add the fish stock, cover with a lid and simmer gently for 5-10 minutes.
Strain the stock through a sieve, pressing the shells with a ladle to squeeze all the juices out. Return the stock to the heat and bring to a simmer.
Continue to cook until the volume of the stock has reduced by three-quarters. Whisk in the butter, a splash of wine and season, to taste, with salt and freshly ground black pepper. Keep warm.
For the lobster tortellini, blend the scallop meat with a good pinch of salt in a blender or food processor until smooth; then add the egg white and blend until smooth. Gradually blend in the cream.
Stir in the lobster meat and herbs and leave to chill in the fridge for 15 minutes.
To make the tortellini, lay a wanton wrapper on a chopping board and place a teaspoon of the lobster mixture in the centre. Brush the edges with the beaten egg and fold over to form a triangle.
Cut out a semi-circle using a 10cm/4in round cutter. Wrap the edges of the semi-cirlce around your index finger and join together with a little more beaten egg.
Repeat with the remaining mixture and wonton wrappers.
To serve, heat three-quarters of the lobster sauce until boiling and cook the tortellini for 3-4 minutes, or until tender. Remove with a slotted spoon and set aside, then add the reserved lobster tail pieces to the sauce and warm through.
Meanwhile, heat the butter in a frying pan and fry the orach (or spinach) for 1-2 minutes, or until wilted. Remove the orach from the pan and set aside. Add the cream to the pan and continue to cook until reduced slightly. Chop the orach, stir it into the cream and season, to taste, with salt and freshly ground black pepper.
Blend the remaining lobster sauce with a hand blender to create a foam.
Place the lobster tail onto serving plates and spoon some orach alongside. Sit the tortellini on a small spoon alongside the orach, drizzle over some of the sauce and the foam and sprinkle over the sea purslane.