Preheat the oven to 190C/370F/Gas 5.
For the chicken, remove the mini-fillet at the back of the chicken breasts and set aside. Using a sharp knife, slice open the breasts so that they can be opened up like a book from the top to the bottom. Gently bash the breasts out with a meat mallet or rolling pin so they are flat.
Repeat the flattening process with the mini-fillets. These little fillets will be used as sticking plasters to stop the butter from oozing during cooking.
For the herb butter, place the parsley, butter, garlic, Dijon mustard and the salt and pepper into a bowl and mix together well. Divide the butter mixture into four thumb-sized pieces and place into the fridge to chill and set.
Turn each flattened chicken breast over and put a roll of herb butter in the centre of each and then roll up like a cigar. Cover any holes with the flattened fillet pieces.
Dredge the breasts in flour and place into the freezer for 2-3 minutes to cool.
Meanwhile beat the egg and milk together in a bowl.
Remove the floured chicken from the freezer and dip in the egg mixture to coat thoroughly.
Roll the breasts in the breadcrumbs, making sure that they are well coated, then place in the fridge for 30 minutes.
Heat 120ml oil in a pan over a medium heat. Add the chicken breasts and fry for two minutes, until golden brown. Turn the breasts and transfer to the oven to cook for 12-14 minutes, until golden and completely cooked through.
Remove from the oven and drain onto kitchen towels.
Serve with mashed potato and green beans, with lemon wedges to garnish.