Recipe for Rep - the vegetable dish of Zambia

To accompany Nshima the Zambian's have a dish called Rep (or Rape) which is a green leaf dish.  I fell in love with this dish and after a few attempts to make it taste right have come down to this recipe:

Ingredients
1 lb of spring greens or cabbage or similar
1 chopped onion

3 chopped tomatoes
100 ml of water
pinch of salt
teaspoon of oil (if required for taste)
Saucepan

Instructions
heat oil in the saucepan.
add the onion and soften until light brown
add the tomatoes and the water and keep cooking
add the green leaves, finely chopped
keep stirring on the heat until cooked - approximately 10 minutes