Step 1In a medium skillet, heat one-fourth cup of the olive oil and saute the onions over low heat until very tender and nearly caramelized, about 25 minutes. Season the onions with salt and pepper to taste.
Step 2Remove the pin bones from the fish using tweezers, pliers or your fingers, or ask your fish market to do it. Cut the fish fillet crosswise into pieces about 1 1/2 inches wide.
Step 3Place the matzo meal in a shallow dish and season with salt and pepper. In a large nonstick skillet, heat about one-half inch of vegetable oil until hot but not smoking. Lightly dust the fish with the seasoned meal, add to the pan, and fry until golden. Carefully remove the fish from the skillet and arrange in a nonreactive baking pan of stainless, glass or enamel.
Step 4Arrange the onions and chopped parsley over the fish.
Step 5To make the marinade, in a small bowl whisk together the remaining 1 cup of olive oil, the vinegar, mustard, shallot, lemon zest and salt and pepper to taste. Pour over the fish, cover with plastic wrap and marinate in the refrigerator for at least 24 hours, and up to three days. Bring to room temperature before serving.
Step 6Arrange the radicchio or butter lettuce on a serving plate. Lift the fish, topped with the onion mixture, out of the marinade and arrange on the lettuce leaves. Garnish with the pine nuts.