Crab in Black Bean Sauce

Ingredients:

1 large crab (or 2 smaller crabs), chopped (ask the butcher to do it unless you feel like getting crab guts all over you!)
1 large onion, cut into pieces (whatever shape and size you prefer)
1/2 cup scallion, cut into lengths
4 pcs thinly sliced ginger
1-2 cloves garlic, chopped
1 pc thinly sliced small red chili

**Black Bean Sauce: (this is for freshly made sauce; there's a ready-to-use version by Lee Kum Kee that's sold in a jar, though I've never tried that one)
4 TBS preserved black beans (kinda sticky and comes in a sealed plastic bag)
garlic
2 TBS sugar
2 TBS soya sauce
1 tsp sesame oil
pinch of ground white pepper
2 cups water

**Light Soya Sauce Water:
1 TBS soya sauce
5 TBS water
1/2 TBS sugar

1. Heat wok with 2 TBS oil, fry onion, ginger, garlic and scallion until it smells (that's in my mom's words).
2. Add the black bean sauce (made by mixing the ingredients indicated above).
3. Add crab, stir fry for about 10 minutes.
4. Cover, making sure the water level almost completely covers the crab. If there's too little liquid, add some light soya sauce water (made by mixing the ingredients indicated above). Cook on medium fire for about 8 minutes.
5. Prepare 1/2 cup cold water with 1 TBS corn starch (= sauce thickener). Pour into wok, stirring constantly, and add 1 TBS cooking wine, till sauce is thicker and crab has turned completely red. Serve.