"'I'll take your word for it,' said Ron, helping himself to black pudding"
(Goblet of Fire 251).
You may, like me, not blame Ron for passing up the bouillabaisse for something more familiar to him. But knowing the ingredients of black pudding makes me wonder which carnivorous "casserole" is more... well... icky.
In The Unofficial Harry Potter Cookbook, Dinah Bucholz includes a recipe for black pudding, but includes the caveat that it is almost impossible to prepare in the United States because of laws regarding certain ingredients.
The recipe requires 3.5 cups of blood (sheep's or pig's is recommended). And unless you happen to be connected in some way to a slaughterhouse, you probably have no way to legally obtain animal blood while residing in the U.S. And there is NO WAY that I'm going to attempt to create a vegetarian version of black pudding, as I have no desire of tasting ingredients to find a substitution for the flavor of blood.
Sorry folks, this is one you'll have to imagine on your own. Or take a trip to Britain.