Skin and trim the fillets of fish and cut across the turbot and salmon to make long slices about 5 mm/¼ inch thick; the skate fillets will be about this depth anyway. Poach the fish separately in a buttered shallow tray with the fish stock and white wine for about 1 minute. When cooked, remove from the liquor and allow to cool. (The stock can be used again in another recipe.)
Blanch the spinach leaves and refresh in ice-cold water, then pat dry between two cloths. Blanch, skin and deseed the tomatoes and cut into small dice. Peel and segment the limes and dice. Soak the leaves of gelatine in cold water until soft. Heat the consommé and, when the gelatine is ready, add it to the consommé and stir until dissolved. Allow to go cold.
Line the terrine mould with the spinach leaves, leaving at least a 5 cm/2 inch overhang all the way round and making sure not to leave any gaps. Pour a 3 mm/⅛ inch layer of the cold consommé into the mould, sprinkle in a little of the tomato, cucumber, chives, limes and peppercorns and allow to set in the refrigerator. When set, pour in a little more consommé and lay in half of the turbot to form an even layer. Leave this to set and then sprinkle it with the tomato, cucumber, chives, limes and peppercorns and add another 3 mm/⅛ inch layer of consommé. Return to the refrigerator and allow to set again. Continue building up the layers like this, alternating the fish and garnish, starting with the turbot followed by garnish, then salmon, garnish, skate, garnish, salmon, and so on. The quantities are enough for 2 layers of turbot and skate and 3 of salmon. Finish off by folding the overhanging spinach over the top of the terrine. Allow at least 6 hours for the terrine to set completely.
For the sauce, mix together the yoghurt and cream, add the lemon juice, seasoning and a little sugar if required; mix well.