Rinse and trim spinach and remove the stems. Pour into a large pot of boiling salted water and immediately pour into a colander, drain well and spread on a kitchen towel.
Chop bacon very finely. Mix parsley with bacon and walnuts.
Rinse fish fillets in cold water and pat dry. Separate the egg. Whisk egg white and brush onto the fillets and season with salt and pepper. Distribute bacon nut mixture on it and roll up the fillets gently.
Wrap each plaice roll in spinach leaves, shaped into a packet and tie with kitchen twine. Place together in a sufficiently large steamer basket.
Boil the fish stock in the steamer pot, position the steamer basket, cover and steam the plaice packets over medium heat for about 5 minutes. Remove gently, cover and keep warm.
Stir creme fraiche into the stock and bring to a boil. Let the sauce boil a little and season with lemon juice, salt and pepper. Remove the pan from the heat and stir in the whisked egg yolks. Arrange plaice packets on plates, drizzle with sauce and serve with basmati rice.