Start by preparing the marinade by mixing the ingredients in a bowl.
In a saucepan, sauté the chopped onion in 2 tablespoons of olive oil over low heat, covered, for 10 minutes. Stir occasionally.Then place the onion in the bottom of your tagine.
Season the chicken thighs with salt and pepper, then place them in the same pan as the onions and brown them for 5 to 10 minutes on all sides, turning them several times.
Place the chicken thighs in the tagine, on top of the onions, and coat them evenly with the marinade.Add the chopped preserved lemon and arrange it around the chicken.
Cover the tagine (or casserole dish) and place it in a cold oven. Preheat it to 160°C/325°F for 1 hour.Remove the cover and return it to the oven for 15 minutes, still at 160°C/325°F.