To teach you an easy way: first lettuce with boiling water hot cooked (do not need to put any seasoning) on a plate to spare. Then boil the soup: put a little oil in the pot, pour oyster sauce and stir fry, then put a little water to boil, taste, according to your taste can be appropriate to add salt, monosodium glutamate and sugar. Then thickening pour to the spare lettuce, so oyster sauce lettuce is ready.
The advantage of doing this is to control the flavor of the soup you can control the flavor of the whole dish.
Note: The soup can be a little salty, otherwise the flavor of the dish may be light.