OK, I’m cheating a bit with this one because it isn’t a flower. It’s a fern. In North America, the ostrich fern is the one used. But there are others that are safe to eat in other places. Please be sure to buy these in the market or to consult a reliable field guide before picking!
Fiddleheads should be washed in several changes of water to remove bitterness. Boil or steam to cook them thoroughly. If you like, you can finish by sauteing in butter. But it’s probably not safe to skip the steaming step.