Cover the bottom of the tagine with a tablespoon of oil, then add the onion rings and place the garlic head in the middle of the dish.
In a bowl, mix the herbs, the spices (saffron excluded) and the oil; sprinkle a couple of spoons of the mixture on top of the onions, and then add the harissa.
Place the tagine on medium heat on a gas stove, add 4 oz. (120 ml) of the vegetable stock and cook for 5-10 minutes, covered.
Clean the vegetables, then cut in long segments.
Cut the rest of the onions, the tomatoes and the potatoes in 1/4 inch slices.
Place the vegetables in the dish, starting with those requiring a longer cooking time. The order that I used started with white turnips, carrots, green beans, white flower gourds, cauliflower, zucchini, and finally, the green peas.
Then, cover the vegetables with the slices of potato, tomato and onions and the confit lemon cut into wedges. Season with a little bit of salt and pepper.
Place the tagine back on the heat and add hot vegetable stock. Cover the tagine.
Every 10 minutes, remove the cover and distribute the sauce over the vegetables. Simmer for 40 minutes.
Wash the olives and distribute them on the tagine a few minutes before you are ready to serve.
Last touch: crush the saffron by hand over the tagine before serving.