Clean, wash and chop the soup greens. Wash fish. Put both with half of the lemon juice and the peppercorns in 3/4 litre salted water, bring to the boil and let it simmer for about 15 minutes at low heat.
Take out the fish, skin and bone it. Cut fish flesh into 3 cm pieces. Sieve the fish stock and bring to the boil again. Rinse clams and let them simmer in the stock for 10 minutes. Clean, wash and cut the leek into fine strips.
Remove mussels from the stock, discard unopened mussels. Finally cook the scampi or crab and leek strips in the stock for 5 minutes. Remove the scampi or crabs and peel them. Heat crab butter in a large pot.
Add the flour and sweat everything. Add 1/2 litre of fish stock while stirring. Bring to the boil. Whisk egg yolk and 4 tablespoons of cream and stir into the hot, but not boiling sauce. Beat the rest of the cream until semi-stiff and fold in.
Season the crab sauce with salt, pepper, sugar and the remaining lemon juice. Briefly heat the fish pieces, mussels and scampi in the sauce. Arrange in a bowl. Serve with crispy white bread.