If ham has a layer of fat, score edges. Place in a shallow baking dish and bake, uncovered in a 300ºF (150ºC) oven for 1 hour, until heated through.
Drain cherries, reserving juice. Add enough water to cherry juice to measure 3/4 cup (175 mL) and heat in a small saucepan.
Combine sugar, flour, mustard and salt; add to cherry juice and cook over medium heat, stirring until thickened. Stir in lemon rind, food colouring, if using, and cherries. Heat through. Pour sauce over ham, and bake 30 minutes more.
To serve, cut ham in slices and spoon cherry sauce over them.
Makes 8 servings, with leftovers.