Saute the ground lamb and onion, breaking up the meat with a fork. Add garlic powder and thyme. Set aside. Preheat oven to 160 degrees C.
Cut the tops off the zucchini and scoop out the insides. Chop up the flesh and add to the lamb mix. Add the couscous and salt and pepper to taste. Stuff the zucchini with this mixture and put the tops back on.
Line a tray with foil and place the zucchini on it. Cover with foil and bake about 2 hours until zucchini is very tender.