Season chicken breasts with garlic powder, pepper, and salt.
Heat 2 Tablespoons oil in a large pan over medium-high heat. 2 pieces at a time, cook chicken until golden on each side and cooked to an internal temperature of 165 degrees F, about 3 minutes per side, depending on the thickness of your chicken. Transfer cooked chicken to a plate and set aside. Wipe pan clean with paper towel, add 2 Tablespoons oil, and cook remaining 2 pieces of chicken. Transfer cooked chicken to plate and set aside. Do not wipe pan out.
Reduce heat to medium-low, add remaining 2 Tablespoons oil to pan, and cook onion until softened, about 1 minute. Add minced garlic and sauté until fragrant, about an additional 30 seconds.
Pour in chicken stock and, using a wooden spoon, scrape up browned bits from bottom of pan. Allow mixture to sit and simmer until it reduces slightly, about 3 minutes. Pour in heavy whipping cream and simmer for about 1 minute. Stir in Parmesan cheese and allow it to melt.
Add cooked chicken back to pan and allow it to simmer to thicken the sauce, about 2 minutes.
Garnish with parsley and freshly cracked pepper, then serve.
