To prepare the shrimp cakes:
Purée the sole in a food processor fitted with the metal blade until smooth. Transfer to a bowl and add the shrimp, scallions, chives, parsley, lemon juice, and mayonnaise. Season to taste with salt, pepper, and cayenne pepper. Form the mixture into 8 patties, cover with plastic, and refrigerate for at least 30 minutes.
Place the flour on a large plate and dredge each shrimp cake in the flour, brushing off any excess. Heat a large sauté pan until just hot. Pour equal amounts of olive oil and clarified butter into the pan until it is approximately 1/2 inch deep. Heat the mixture over medium-high heat for 5 minutes, or until very hot. Carefully add the shrimp cakes to the pan and sauté for 3 minutes, or until golden brown. Carefully turn the cakes over and cook for 3 minutes, or until golden brown. Remove the cakes from the pan and drain on paper towels.
Spoon a mound of the salsa in the center of each plate and lean 2 shrimp cakes against the salsa, opposite each other. Place a sprig of chervil in the center of the salsa.