Heat the oven to 190C/fan 170C/gas 5. Put all the stew ingredients in a large casserole, season and stir well. Put a sheet of foil under the lid to create a tighter seal, then firmly push the lid down. Cook in the oven for 1 hour. Meanwhile mash the flour and butter together until a smooth paste. After an hour of cooking, stir the flour mixture into the stew and put back into the oven, uncovered, for another 30 minutes-1 hour or until the lamb is tender and the stew has thickened. Stir in a few fresh thyme leaves, and serve with mashed potato and peas.