Place sliced onions and bell peppers in the bottom of your slow cooker.
Lay the frozen chicken breasts over the vegetables.
Mix together the brown sugar, vinegar, Worcestershire sauce, chili powder, ketchup, and soy sauce. Pour over chicken.
Top the chicken with pineapple chunks.
Cover and cook on low for 6 to 7 hours.
Remove chicken and vegetables. Keep warm.
Remove about 1 cup of the liquid and stir in the cornstarch. Add the mixture back into the crock pot and increase heat to high. Allow to cook uncovered, stirring, until slightly thickened.
Place chicken and vegetables back into the slow cooker. Toss with sauce and serve.