In a big pot, boil the chopped onion and tomato with a little water. After it boils for a while, put in the meat and add water to almost cover. Add the fish sauce and peppers, bring to boil, reduce heat and simmer till meat is tender and cooked thru (couple hours--look up cooking times depending on whether it's closer to 2 lbs or 3). Chop meat and add back to pot. Add radish, beans, and greens. Bring to boil, reduce heat, cover and simmer till the veggies are tender (mustard greens take a while). Before serving, mix in the tamarind paste. Serve in soup bowls over cooked rice with Sriracha hot sauce.