For stuffed onions, bring the soup or salted water to a boil. Put in the onions and simmer for about 8 minutes. Remove and let cool slightly. Peel and cut in half. Scoop out the inner layers, chop coarsely and set aside for the filling.
For the filling, cover couscous with an equal amount of boiling water. Cover and let swell for 10 minutes. After 5 minutes, loosen with a fork.
Peel the celery slice and chop into small cubes. Pluck the thyme leaves from the stem and chop finely. Peel the apple, cut it in half and remove the core and the stem. Cut into small cubes. Immediately mix with the other ingredients to prevent the apple pieces from turning brown.
Fill the hollowed out onions with the mixture. Tear or cut aluminum foil into rectangles and place the stuffed onion halves on top. Twist the foil together over the onion halves and grill the packets on the hot grill for about 15 minutes. Unwrap stuffed onions from foil and arrange.