Combine the pork, onion, ginger, tomatoes, fish sauce, salt, black pepper, and water in a 6-7 quart heavy bottom pot or Dutch oven and put over medium-high heat.
When the mixture reaches a boil, reduce the heat to medium-low. Add the tamarind and the citric acid.
Partially cover the pot with a lid and simmer for 90 minutes, stirring occasionally, until the pork is tender.
Add the daikon and simmer another 20 minutes.
Add the bok choy and cook a final 10 minutes.
Serve in deep bowls over freshly cooked white rice.