Place butter, garlic, lemon juice and parsley in a bowl. Season with salt and pepper. Mash with a fork to combine. Spoon mixture into the centre of a 20cm piece of plastic wrap. Roll up to enclose butter mixture, twisting ends to seal. Roll on a board to form a 10cm-long log. Refrigerate for 1 hour or until firm.
Using a sharp knife, make a deep cut along 1 side of each chicken breast to form a large pocket (do not cut all the way through). Remove plastic wrap from the butter. Cut butter into thick slices and place 4 butter slices into each pocket.
Place breadcrumbs on a large plate. Season with salt and pepper. Place the flour on another plate. Whisk eggs in a shallow bowl. Coat 1 piece of chicken in flour, shaking off excess. Dip in the egg mixture and coat in breadcrumbs or panko. Place on a plate and repeat with the remaining chicken… flour, egg and breadcrumbs. Preheat oven to 180°C.
Pour enough oil into a large frying pan to cover the base. Heat over medium-high heat. Cook chicken, in batches, for 2 to 3 minutes each side or until golden. Finish cooking in the oven for 10-15 minutes or until cooked through. Serve with mashed potatoes and green beans or salad.