In a medium sauté pan over medium heat, heat oil, one turn of the pan. Add mushrooms and brown, around 6-8 minutes. Add in onions and garlic, and sauté until tender, around 2-3 minutes.
In a 6.5-quart slow cooker, place the mushroom mixture, chicken breasts, cashew butter, white wine, chicken stock, Worcestershire sauce, thyme, salt and pepper. Set it on high for four hours.