Mix panko, onion powder, rosemary and salt in another bowl or dish.
Set each chicken breast on a cutting board. Set one palm on top of one breast to hold it still then use a chef’s knife with the other hand to slice the breast in half horizontally. Repeat with the rest of the chicken breasts, then, using a meat mallet or the bottom of a heavy skillet, pound each piece until no more than ⅓-inch thick. Season generously on both sides with salt.
Put olive oil and butter into saucepan and bring to temp on medium flame.
Thoroughly coat all chicken in flour then dredge through the egg mixture and allow excess egg to drain off before placing in bread crumb mixture and ensuring each piece is throughly coated, place into the butter/olive oil mixture and cook until golden brown on each side, roughly 5 minutes per side.
Garnish with squeeze of lemon and flat leaf Italian parsley.