Complex sentence: Black ovens are heated by burning wood in a chamber and the food is cooked in that same chamber alongside the fire while it is still going, or in the heated chamber after the fire and coals have been swept out.
Split into simpler sentences: Black ovens are heated by burning wood in a chamber. Food may then be cooked in that same chamber alongside the fire while it is still going, or in the heated chamber after the fire and coals have been swept out.