'Tiella' is simply a dialect word for tegame, a pan, and as a dish it can mean many things in different parts of Italy. Tiella Barese is a Puglian classic, a sort of paella made of layers of tomato, onion, rice, mussels and sliced potato, and then baked (it's often called tiella al forno, or even tiella di cozze).
In Lazio, a tiella di Gaetá is a very thin pie with upper and lower sheets of dough. It may have calamari inside, like a Languedocien tielle in France, or else broccoletti or other vegetables (these often appear in pizza à taglio shops).
Some tielle are merely casseroles without the crust, like the tiella Abruzzese, made with layers of potatoes, onions, tomatoes, aubergines and peppers. Elsewhere in the south tielle might have layers of potato, mushrooms and pork.
Images by: Letizia Palmisano, Creative Commons License