In a microwave-safe glass container or bowl, add the fish balls, shiitake mushrooms, tofu, sliced beef, bok choy, and napa cabbage. Spread everything out evenly and gently tucking so they are snug without overcrowding.
In a separate bowl, combine the soy sauce, sesame oil, garlic, ginger, chicken broth, and dried red chilies (if using). Stir well, then pour the broth evenly over the hotpot ingredients.
Cover the container with a microwave-safe plate. Microwave on high for 6 minutes or until the beef is cooked through and the vegetables are tender.
While the hotpot cooks, combine the soy sauce, chicken broth or water, chili oil, rice vinegar, sugar, garlic, chopped green onion, and sesame seeds in a small bowl. Stir until the sugar dissolves.
Carefully remove the hotpot from the microwave (it will be hot) and remove the plate. Enjoy immediately with the dipping sauce on the side.