Preheat oven to 190°c - 375°f.
Melt the clarified butter in a large ovenproof pan over medium heat, add in the minced garlic & sauté for 1 minute or until fragrant. Add in the mushrooms, seasoning to taste & continue to sauté for 5-6 minutes or until the mushrooms are soft and golden. Add in the flat-leaf parsley, mix through & remove from the heat.
Season the prepared chicken breasts with onion powder, garlic salt & cracked black pepper, rub to coat.
Place the sliced bocconcini balls into the slit of the breast, divide the mushroom mix between the two breasts, reserving some mushrooms & the juice for the sauce in a bowl. Grate over parmesan cheese to your taste to cover the slit In the breast then seal the chicken using toothpicks or skewers.
Set the pan that was used to cook the mushrooms back over high heat (make sure the mushrooms have been removed and the pan is empty). Add in the olive oil & sear the stuffed breasts for 3 minutes on each side until golden.
Place the pan into the oven & roast for 13 minutes (this will be perfectly cooked). Remove from the oven and allow to rest for 10 minutes.
In the meantime place a pan over medium heat. Add in the clarified butter & allow it to melt. Add in the garlic, reserved mushrooms & juices, sauté for 1 minute. Pour in the full fat cream, season to taste & bring to a simmer. Once simmering, allow it to continue to do so for 7 minutes or until reduced & thick, stirring frequently.
Add in the roughly chopped flat-leaf parsley, the pan juices from the chicken, parmesan cheese and check for seasoning. Stir to combine and melt the cheese, remove from the stovetop.
Remove the toothpicks or skewers from the chicken breast, place onto a plate, pour over the mushroom sauce & serve alongside a salad of your choice. Dig in.