Preheat oven to 200°C or 400°F.

Melt butter in a large oven proof pan or skillet over medium heat. Add garlic and sauté until fragrant (about 1 minute). Add in mushrooms, salt and pepper, and parsley. Cook while stirring occasionally until soft. Set aside and allow to cool while preparing your chicken.

Pat breasts dry with a paper towel. Season with salt, pepper, onion powder and dried parsley. Rub each piece to evenly coat in seasoning.

Horizontally slice a slit through the thickest part of each breast to form a pocket. Place 2 slices of mozzarella into each breast pocket.

Divide the mushroom mixture into four equal portions and fill each breast with the mushroom mixture. Top the mushroom mixture with 1 tablespoon of parmesan cheese per breast. Seal with toothpicks near the opening to keep the mushrooms inside while cooking.

Heat the same pan with the pan juices. Add the chicken and sear until golden. Flip and sear on the other side until golden. Cover pan and continue cooking in preheated oven for a further 20 minutes, or until completely cooked through the middle and no longer pink.

Serve, with pan juices and any remaining mushrooms, on top of pasta, rice or steamed vegetables.

(To make the optional cream sauce, transfer chicken to a warm plate, keeping all juices in the pan.)

Fry the garlic in the leftover pan juices until fragrant (about 1 minute). Reduce heat to low heat, and add the mustard and half and half.

Bring the sauce to a gentle simmer and add in any remaining mushrooms and parmesan cheese. Allow the sauce to simmer until the parmesan cheese has melted slightly. If the sauce is too runny for your liking, add the cornstarch/water mixture and mix through fast to combine into the sauce. It will begin to thicken immediately.

Season with a little salt and pepper to your taste. Add in the parsley and the chicken back into the pan to serve.