Cut each chicken breast in half all the way through to give two escalopes. Put a layer of parchment paper on the work surface and pop the pieces of chicken on top well-spaced apart. Cover with another piece of parchment and, using a rolling pin, bash the chicken until each piece is 2-3mm thick.
Spread the mustard on each piece of the chicken. Arrange a slice of cheese on the bottom pieces followed by a slice of ham and a slice of cheese. Cover with the top pieces and press down so that the filling is completely enclosed – trimming down the edges if necessary.
Dip each one in flour, then beaten egg, then press in the breadcrumbs to coat evenly. These can be put on a plate and covered with cling film and kept chilled for up to 2 days.
When ready to cook, heat a large non-stick frying pan over a medium heat. Add the oil and butter and once the butter is foaming add the coated chicken breasts and cook for 5-6 minutes on each side or until cooked through and golden brown, reducing the heat if you think they are browning too quickly. Arrange on warmed plates with a dollop of redcurrant jelly and the green beans and mashed potatoes, if liked to serve.