Melt some oil in a frying pan over a medium-high heat. Add your chicken breast chunks, season them and fry for about 5 mins until cooked and starting to go golden. Set aside.
In a wide saucepan, melt your butter. Add your onion and fry for about 5 mins until soft. Add your garlic and cook for a further 1 min. Put your flour in the pan, stirring so it fully coats the onion and cooking for one minute. Gradually add the chicken stock, stirring all the while, then simmer for 5 mins until thickened.
Add your cream, simmer for another 2 mins, before putting your chicken, ham, gruyere, parmesan and mustard in the pan. Stir to combine, then finish with your parsley, salt and pepper. Set aside
Mash your potatoes, then season with salt and pepper. Take a handful and flatten in your hand, then stuff with a cube of cheese and roll into a ball.
Pour your chicken filling into a rectangular baking dish, then sprinkle with your mozzarella. Top with your mash balls, then bake for 30 mins.