Preheat the oven to 200°C (180°C Fan/Gas 6).
Toss the potatoes, onions, garlic, olive oil, salt, pepper, and chopped ham in a roasting tin.
Roast for 25 minutes until the potatoes begin to soften.
Wrap each chicken breast with the remaining Parma ham and season.
Remove the roasting tin from the oven.
Stir in the mushrooms, white wine, double cream, and Reblochon cheese.
Place the wrapped chicken on top of the vegetables.
Return to the oven and bake for 20–22 minutes until the chicken is fully cooked.
Remove the chicken and allow it to rest for 5 minutes.
Stir the mustard, lemon juice, thyme, and parsley into the vegetables and sauce.
Slice the chicken and serve over the creamy potato mixture.