it is a lamb dish from the Cyclandes in Greece.
Mastello is named for the large clay pot the lamb is cooked in over the vinewood.
I believe it is an easter specialy on Sifos.
It is made with potatoes,red wine,dill,olive oil Etc.
people used to bring it to the bakers to cook it for them.
Mastello is also a cheese from the northeastern aegean.
I hope this helps.