I think if you sear the meat on the range in a skillet, then saute the veg and put it all in the Romertopf and into the oven, it will probably turn out pretty well.
Be sure to soak the Romertopf in warm water for awhile just before using it in the oven.
If you scroll down a bit on this link there’s a tagine recipe for the Romertopf. https://www.kitchenkapers.com/pages/focus-on-the-romertopf-roaster
It sounds so, so good.
Yesterday on Foodnetwork, Chef Maneet didn’t have a tagine so used a skillet to sear the meat.
I don’t have a Romertopf, per say, but do have a ceramic dutch oven which looks to be the same animal.