Lightly rub a little oil over each chicken breast. Place between 2 pieces of cling film and bash with a rolling pin until flattened and no more than 1cm thick. Season and place a slice of ham on top of each chicken breast. Mix together the cheese, crème fraîche and mustard and spread over the ham. Cook or wrap well and freeze.
To cook straight away, place the chicken on a lightly greased baking sheet and cook under a medium grill for 7-10 mins until chicken is cooked through and the cheese is bubbling and browned. If the cheese browns a little too much for your liking, simply finish off the cooking in a hot oven.
To cook from frozen, heat oven to 180C/160C fan/gas 4. Place the chicken on a baking tray and loosely cover with foil. Cook for 30 mins, then remove the foil, increase the temperature to 220C/200C fan/gas 7 and cook for a further 25 mins.
If you want the topping golden, flash under a hot grill for a few mins. Serve in a crusty roll with some green leaves, tomatoes and coleslaw on the side.