In a shallow dish or bowl, combine flour, garlic powder, onion powder, salt and pepper and stir to combine.
Dip each chicken breast into the flour mixture and cover it completely. Shake of any excess flour mixture and set aside.
Heat olive oil in a skillet over medium heat on the stovetop. Add chicken, working 2-3 pieces at a time, and cook for about 2 minutes on each side until browned.
Add sliced onions to the bottom of the crock pot and lay the browned chicken over the top.
Add the chicken broth, marsala wine, mushrooms and garlic to the top and cover and cook on high for 3-4 hours or on low for 6-8 hours until chicken is tender and cooked through.
Stir in heavy cream and cover and cook an additonal 30 minutes and then serve.
