Slice the chicken in half lengthwise, as if you are butterflying, but slice all the way through. Season the thin chicken breasts with garlic powder, salt, and pepper, and set aside.
In a large skillet, heat the butter and oil over medium heat. When the butter is melted, cook the chicken on each side for 5-6 minutes until golden brown and cooked through. Set the chicken aside.
Add the mushrooms and garlic to the pan. Saute for 3-4 minutes until the mushrooms are golden brown.
Add the marsala wine to the pan, scrape the brown bits off the bottom of the pan, and cook the wine over medium heat until the wine is reduced by half (about 4-5 minutes).
Add the chicken broth and heavy cream and bring to a simmer. Slowly add the cornstarch slurry, stirring constantly until well combined. Simmer for 2-3 minutes, stirring occasionally, until the sauce is thickened.
Add the chicken back to the pan and toss in the sauce. Garnish with thyme leaves and serve!