Mix the cornstarch and the 3 tablespoons of water in a medium bowl, until the cornstarch dissolves. Add the soy sauce, brown sugar, pineapple juice, garlic powder, and black pepper. Mix well. Put the mixture into the slow cooker.
Add the chicken, garlic, mushrooms, green onions, and sesame seeds into the slow cooker, making sure to save some green onions for serving.
Give all the ingredients a stir.
Cook on low for 8 hours.
Stir once more after cooking time is done.
Serve with rice, and top with reserved green onions and sesame seeds.