Blitz the walnuts in a food processor until they resemble breadcrumbs. Transfer to a bowl, mix in the remaining filling ingredients and set aside.
Place all the syrup ingredients except the rosewater in a pan. Gently bring to the boil and simmer until the syrup has thickened and coats the back of a spoon lightly (like single cream). This should take around 30 minutes. Remove from the he