So we've made three tagines now, all within a week or so. I know I promised a recipe, but in working with this I've come to realize that tagine is not so much a recipe as a technique. I file it under the same kind of dish as paella or risotto: Dishes that have an equation of ingredients and a more or less set process. If you stick to the equation, and follow the process, you are free to play around with the ingredients. There's no reason why you couldn't make a tagine that has overtly Italian, Indian or Thai notes just by swapping in the appropriate spices and ingredients. The net result will be as satisfying.
Basically, tagine is simply a hybrid between a braise and a stew, comprised of meats (which are in fact optional), aromatic and/or sturdy vegetables (also optional, if you want to make an all-meat version), fruits (optional, but very nice), spices/herbs (decidedly not optional) and liquid (fundamentally necessary). And heat.
The real challenge is liquid management. You'll need less than you think. As the dish cooks, the ingredients will give off some of their own liquids; as the steam builds under the cone, pressure will force the liquid down, which in turn pushes it out around the periphery of the lid. Less is more.
All tagines -- the cooking vessels themselves -- are different. I can only speak for the Le Creuset version. Traditional ceramic ones may be more shallow; the All-Clad version appears deeper. But in our experience so far, the tagine equation is as follows: