1. Cut tomatoes horizontally in half, squeeze with hands and discard juice and seeds. Chop 1 tomato in small dices, sprinkle with salt and let drain over a colander for 30 minutes. Purée the rest of the tomatoes. Sprinkle with salt and let drain in a colander.
2. In a bowl combine cream, mustard, vinegar, cognac, salt and pepper. Beat well until thick. Add the tomato purée, parsley and tarragon. Beat well and stir in the chopped tomato. Refrigerate.