Put oven rack in middle position and preheat oven to 200 degrees F.
Combine fish, half of the lemon slices, wine, and 1/4 teaspoon salt in a shallow dish. Marinate, covered, at room temperature 30 minutes.
Meanwhile, whisk together egg, milk, and another 1/4 teaspoon salt in a shallow bowl.
In another shallow bowl, stir together bread crumbs, garlic, parsley, pepper, remaining 1/4 teaspoon salt.
When fish is done marinating, dip fish pieces, one at a time, in egg mixture, letting excess drip off, then dredge in bread crumb mixture, shaking off excess. Transfer to a sheet of waxed paper, arranging fish in one layer.
Heat 2 tablespoons oil and 1/2 tablespoon butter in a 12" non-stick skillet over moderately high heat until hot but not smoking. Fry 2 pieces of fish, turning over once, until golden and just cooked through, 4-5 minutes total. Transfer to a baking sheet and keep warm in oven, then cook remaining 2 pieces of fish in remaining 2 tablespoons oil and the remaining 1/2 tablespoon butter in the same manner.