Soak gelatine in cold water. Wash the fish fillets and sprinkle with lemon juice. Heat the stock. Season with salt, pepper and vinegar. Let fish fillets simmer in the hot stock for about 5 minutes.
Then set aside. Squeeze out the gelatine and dissolve carefully in the warm stock. Clean, wash and, depending on size, halve or quarter the tomatoes. Cut lemon, corn and cucumber into slices.
Place the prepared ingredients on the plate and pour in the stock. Place in the refrigerator for a few hours. Serve garnished with chervil. Serve with baguette or fried potatoes.