Wash the plaice fillets, dab dry and sprinkle with lemon juice. Leave the fish to stand for a short time. In the meantime peel the shallots for the marinade. Bring vinegar, 1/4 litre of water, shallots, peppercorns, mustard seeds, chilli and salt to the boil and cook over a medium heat for 5-8 minutes.
Wash the tarragon and dab dry. Pluck off the leaves, except for something to garnish, and add to the hot marinade. Leave to cool. Mix flour and 1 teaspoon of salt and turn the plaice in it. Shake off excess flour.
Heat 1 tablespoon of fat in a pan and fry the plaice fillets in 3 portions until crispy brown. Put them on kitchen paper and let them cool down. Place the plaice in a flat, wide form and pour the vinegar stock over them.
Leave covered for at least 24 hours. Serve garnished with fresh tarragon.