Heat your oven to 190°C/Fan 170°C/Gas 5. Peel and thinly slice the onion. Trim and slice the leek into rings around 1cm thick. Trim the ends off the celery and thinly slice. Layer the onion, leek and celery in a casserole dish and season well.
Slice the chicken into long, finger length pieces. Warm 1 tsp oil in a frying pan over a medium heat. Fry the chicken for 8-10 mins, turning a few times so it browns all over. Lay the chicken on top of the veg in the dish.
Sprinkle 1 tsp of the sumach over the chicken.Finely grate the lime zest and scatter that over too.
Trim and peel the carrot, then cut into thick slices on the diagonal. Halve the pepper. Scoop out the seeds and white bits and thickly slice. Halve the squash and scoop out the seeds. Peel and chop it into chunks around 2cm thick.
Tumble the veg together in a bowl with plenty of seasoning. Heap the veg on top of the chicken.
Juice the lime. Mix the lime juice and the chopped tomatoes in the bowl. Pour over the veg. Cover and bake for 30 mins till the chicken is cooked through and the veg are tender.
Give the tas kebab a stir, then spoon into warm bowls. Garnish with parsley leaves to serve.