Place sliced onion rings on the bottom of the slow cooker. Arrange chicken on top. Add cumin, salt and pepper, and chicken broth. Cover and cook on low 4-6 hours.
Remove chicken and onions from slow cooker with tongs, removing skin and bones as you transfer to a separate bowl. Using the tongs, toss chicken to break it apart.
Heat beans in a small saucepan over medium-low heat.
Assemble tostadas by spreading thin layer of beans on tostada, topping with chicken and optional toppings.