Step 1For the ravigote sauce: In a small bowl, stir together the mayonnaise, tarragon, parsley, chervil, chives, capers and shallot. Add 2 teaspoons lemon juice, or to taste. Season if desired with salt and white pepper.
Step 2Season the fish with a pinch each of salt and black pepper on both sides. Dredge the fish fillets lightly in the flour, then in the egg mixture and finally in the panko, coating well. Set aside on a rack.
Step 3Add enough oil to come about one-half inch up the sides of a large skillet. Heat over medium-high heat until hot enough that the fillets sizzle gently when placed in the oil. Cook the fillets until crisp and browned on the bottom, about 2 minutes. Then flip them over and cook until they are just done, 2 to 3 minutes longer, depending on the thickness (a slim knife should slide easily into the thickest part). Serve at once, with the ravigote alongside.