you can cook in a clay pot by filling it with soup then dropping hot rocks from the bed of thee fire in there or by burying it in sand under you fire where it receives gradual heat. heat clay quickly or too hot and it breaks and unless glazed it cant be cleaned.
I think there is a reason clay cooking was always abandoned when cast iron became available.
I can cook a four year old hen in the schlemmertopf and the meat is tender and juicy. No need to boil the life out of an old bird when you can just roast in the schlemmertopf for a couple hours.
The most important thing is to soak before using and allow complete cooling before washing.
I bought my first one in Germany almost 30 years ago. Just put your meal (meat, veggies, wine, water, flour, spices) in the pot, bake at about 350-375 for a couple hours, then take off the top and serve right out of the pot.
Use a seperate baker for sweet stuff. The schlemmertopf makes such wonderful moist cobblers and puddings.
I have cracked one over the years. I found another at a garage sale about 6 years ago. It looks like dammit (stains) but makes the best veggie and meat roasts and stews.