Cut chicken fillets into two thin halves by slicing horizontally through centre. Place each piece between two sheets of plastic wrap and flatten slightly with a rolling pin. Toss in seasoned flour, shaking off excess. Dip each piece in egg and coat thoroughly with breadcrumbs.
Heat half of oil and 15g of butter in a frying pan on medium high. Pan-fry chicken in batches for 4-5 minutes, turning halfway and adding remaining oil as necessary, until golden on both sides. Drain on paper towel. Transfer to a heated platter.
Melt remaining butter in same pan. When bubbling, add lemon and capers, shaking pan to combine. Spoon lemon, capers and sauce over chicken and serve with mashed potato.