1. In a bowl, mix cornstarch, chili powder, five spice, and salt. Add pork and mix to coat.
2. Pour oil into a 4- to 6-quart pan over medium-high heat. When hot, add pork mixture and stir often until browned on all sides, about 4 minutes. With a slotted spoon, transfer pork to bowl. Add green onions, chilies, garlic, fermented black beans, and ginger to pan. Stir until fragrant, about 1 minute.
3. Return pork to pan and add 1 cup water, kidney beans, pinto beans, and tomatoes. Bring to a simmer, then lower heat, cover, and simmer (do not let boil), stirring occasionally, until pork is tender when pierced, about 25 minutes.
4. Ladle into bowls and top with sliced green onion tops, if desired.