Put all the tagine ingredients except the grapes in a large casserole, add enough water to cover and bring to the boil.
Turn down to a very low simmer and cook slowly until the mutton is tender (about two and a half hours).
Add the grapes five minutes before the end of the cooking time.
To make the barley, cook the shallots and garlic in the oil over a low heat until soft and light brown. Add the barley and allspice and then cover with water. Simmer for about 15 minutes until tender.
Serve the tagine and barley together, with coriander and pomegranate seeds scattered over.