Blend one of the jarred peppers with ¾ cup of the tzatziki and a pinch of sea salt and black pepper. Pour over the chicken and leave to marinate for at least 2 hours, preferably overnight.
Preheat the oven to 350°F. Peel and quarter the onions, place them in an 11-inch non-stick ovenproof frying pan with 1 tbsp of olive oil on a medium heat for 10 minutes, or until dark and curled, turning regularly, then remove to a plate.
Drizzle 1 tbsp of olive oil into the pan, then add the marinated chicken and brown for 10 minutes, turning regularly. Place the onions back in the pan, rip up remaining peppers by hand then place in pan. Put pan in oven and roast for 25 minutes.
Meanwhile, in a bowl, cover the couscous with boiling kettle water, season with salt and pepper, and cover. Leave to hydrate for 3 minutes, then fluff up with a fork.
After the 25 minutes are up, remove pan of onions and peppers from oven. Add cooked couscous to pan and carefully pat down to compress.
Roast for another 10 minutes, until top is lightly browned, then confidently and carefully turn out onto a large plate or platter and top with dollops of the remaining tzatziki. Serve straight from the oven and let everyone dig in.