I once searched for a recipe and came up dry -- it seems to be limited to commercial sources, and no one has been clever enough to do a knock-off.

Zatarain's is a popular brand of Creole mustard. The ingredients are water, vinegar, brown mustard seed, salt and xanthum gum. Some people take regular brown mustard and add Worcestershire sauce and Tabasco to it. I usually have Zatarain's in the house, but if I don't I'll substitute a coarse grainy Dijon mustard. I've never made it, though. Hope this helps.