Place the chicken cut-side down, cover, and gently pound each piece to an even 0.8 cm / 1/3-inch thickness.
Whisk the egg, mayonnaise, flour, milk, seasonings, salt, and pepper. Turn the chicken through the mixture; cover and marinate for 1 to 24 hours if time allows.
Heat the oven to 200°C / 390°F, spread panko on a lined tray, spray generously with oil, and bake about 12 minutes, stirring halfway, until deep golden. Mix with the crumb salt.
Increase the oven to 220°C / 425°F. Put an oiled wire rack on a tray.
Let excess dredge drip from each chicken piece, press firmly into the toasted panko, and arrange on the rack. Spray the tops with oil.
Bake for 13 minutes without flipping, until the chicken is cooked through and the coating is crisp. Serve immediately with salt flakes and lemon.