How to tell a good oyster omelette from a bad one

That an oyster omelette needs plenty of oysters should go without saying, but try telling that to some cooks. On a cold day in Shanghai, we tried a version of this snack in a pedestrian area that was packed with street-food vendors. The lack of a line at this shop should have told us it wasn't going to be good: the omelette contained a lot of starch but hardly any egg and very few oysters. At least it was cheap, so we didn't feel guilty about throwing it away.

There are several ways to make an oyster omelette, which, at its most basic, consists of oysters and eggs. The oysters should be small - about 3cm or less, so they're left whole, rather than chopped. Most cooks add sweet potato starch, which gives the dish a stretchy consistency. Chiu Chow oyster omelette is flavoured with fish sauce and white pepper. Some versions are more like scrambled eggs with oysters, but they can also be fritter-like, or a puffy, unfolded omelette. In some places, the omelette is topped with a tomato-based sauce (I suspect they use ketchup), which (to me) is undesirable, because it obliterates the flavour of the oysters.