Mix all the filling ingredients together.
Scrape the skin off of the zucchinis using a knife or a serrated peeler. Cut them into thick round slices, empty out the inner part (leaving a thick margin of about 1 centimeter) and add the filling.
Mix the spices with the oil in a small bowl and set it aside.
In a large pot, fry the onions with a pinch of salt until golden.
Add the stuffed zucchinis to the pot. When you're done, turn them over starting with the first one you added to the pot.
Sprinkle the celery leaves and chard leaves on top and pour the spice mixture over the zucchinis. Cover with boiling water, put the lid on and cook on medium-high heat for 10 minutes.
Lower the heat to medium-low, baste the zucchinis with the sauce using a tablespoon and cook for 1½-2 hours, or until the zucchinis are nice and tender.
Uncover and cook for 10-15 minutes to thicken the sauce, stirring occasionally.
Add 2-3 stuffed zucchinis to a deep plate, pour over a ladle full of sauce and serve.