Place chicken breasts in the bottom of a 7-quart greased slow cooker. Top chicken breasts with the onion, beans, green chilies, chicken broth, chili powder, cumin, pepper and salt. Cover with lid. Cook on low heat 4 to 6 hours, or until chicken is thoroughly cooked and tender.
Carefully remove chicken breasts from slow cooker and place on a cutting board. Set aside.
In a medium saucepan, over medium heat, melt butter. Once melted, add the flour. Whisk constantly to combine with the butter. Pour milk in slowly, while continuing to whisk until mixture is thick, creamy and combined. This should take about 5 minutes or less. Pour this mixture into the slow cooker. Stir.
Shred chicken breasts then place back into the slow cooker. Stir gently to combine. Cover with lid and cook another 20 to 30 minutes on low or until chili has thickened. Serve white chicken chili in individual bowls with desired toppings. Enjoy!