Split chicken breasts, lengthwise, into 4 fillets. Season both sides with salt and pepper. Add flour and onion powder to a shallow bowl. Dredge each chicken fillet in the flour.

Heat a large skillet over medium heat. Add butter and oil to a pan. When the butter has melted, add the chicken fillets. Cook until golden brown, about 4-5 minutes on each side. Transfer chicken to a plate and set aside.

Add garlic and mushrooms to pan, and saute until tender.

Stir in chicken stock and let it come to a simmer. Scrape the pan to remove all the browned bits from the bottom of the pan.

Turn off heat, then add in heavy cream and mix. Transfer the chicken back into the pan, and heat on medium heat for another 2 minutes, until the dish is hot. Spoon mushroom sauce over each piece before serving.