Cut off and reserve the stem ends of the zucchini and, with a corer, hollow out zucchini, being careful not to pierce skins and leaving ¼ inch thick shells.
Chop zucchini flesh and reserve.
Heat oil over moderately-high heat until hot but not smoking and saute lamb with allspice and mint and season to taste.
Cook, stirring, until no longer pink.
Transfer lamb with a slotted spoon to a sieve to drain.
Pour off all but 1 tblsp fat from skillet and saute onion until softened.
Stir in garlic, reserved zucchini, season to taste, and saute until golden.
Remove from heat; stir in lamb, pine nuts and 6 tblsps of yoghurt.
Let cool.
Preheat oven to 350 degrees F.
Stuff zucchini with this mixture, putting stem ends back on and securing them with toothpicks.
Arrange zucchini in a large, oiled casserole, tent with foil, and bake for approximately 1 hour, or until tender.
Let cool slightly.
Whisk together remaining yoghurt with cornstarch/water mixture and cook over low heat, stirring, until thickened.