1. Trim excess fat from meat and slice thinly (which is easier when meat is slightly frozen) against the grain.
2. In a large bowl, combine the chopped onion, sliced meat, 1/2 teaspoon of the black pepper, 2 teaspoons of the fish sauce, and the cornstarch; stir well to coat and set aside to marinate (the cornstarch tenderizes and makes the meat velvety ).
3. Over high temperature, heat a large skillet or wok and - when the skillet is very hot - add the oil ( hot wok, cold oil, food won't stick ); add the marinated meat mixture and sauté until the meat is cooked, about 5 to 7 minutes.
4. Remove cooked mixture from skillet and set aside.
5. Heat the remaining 1 tablespoon oil in the pan and stir fry the minced garlic for 1 minute.
6. Add spinach leaves and stir fry for 3 minutes, stirring constantly.
7. Add remaining 4 teaspoons fish sauce and tomato, and stir fry 2 minutes.
8. Add the cooked meat mixture and remaining 1/2 teaspoon pepper to the skillet, combine well, and cook until heated through and sauce comes to desired consistency (the cornstarch will effect the consistency - you can add a bit of water, broth, or sherry if it becomes thicker than you'd like).
9. Serve immediately with hot cooked jasmine rice.
10. Note: I've been told that the recipe is not exactly Vietnamese, since we actually use minced pork to stir fry water spinach; beef is always more expensive than pork at home - the good cuts are often on their own, or cooked with minimal side ingredients; other cuts go into stews and soups so feel free to try this with pork, too!
11. Also, when I first made this recipe I really wasn't expecting too much from it - it came out much tastier than I expected, an absolute keeper (especially according to my husband)!