Put green onion and ginger inside the chicken. Bring the water to a boil and then put the chicken into the boiling water. Turn down the heat so that the water temperature keeps at 100 degree celcius. Cooking time really depends on the time of the chicken, but normally it takes about 45 minutes (+/-). And yes, you can use the water as stock afterwards.
I would recommend that you try Soy Chicken, boiled with a liberal dose of soy sauce. You can get recipes from everywhere, including the internet. When eaten cold it's almost undetectable from roast chicken, other than the fact that the fat in the chicken is easily identified and removed. It's a much healthier choice than other methods of cooking chicken.
Here's one I plucked from the internet earlier:-
1/2 cup vinegar
1 head of garlic, crushed
1/2 teaspoon black peppercorns
1 bay leaf
1/2 tablespoon salt
2 1/2 tablespoons soy sauce
1/4 cup water
1 chicken - cut up
In your slow cooker combine vinegar, garlic, peppercorns, bay leaf, salt and soy sauce. Add chicken. Cover and cook on high for one hour. Add water. Lower heat setting to low and cook for 4 to 5 hours.
I like the sound of the soy chicken - a bit of extra flavour but still healthy - only prob is I don't have a slow cooker - do you reckon I could just simmer it in a normal covered pot instead?
Yes, you can just use normal pot. No slow cooker is needed. If you want to make soy chicken sauce by scratch, be sure to add some sugar as it actually tastes a little (but not very) sweet. You may try to buy a serving of soy chicken from any Chinese restaurants or ParkNShop/Wellcome (from their cooked food section) to get a taste of what it's like before you make it yourself.