Preheat the oven to 350 degrees Fahrenheit.

Arrange the halved potatoes, celery, and sliced mushrooms evenly across the bottom of a large baking pan.

Place the spatchcock chicken on top of the vegetables.

Sprinkle the onion powder, za’atar, turmeric, sumac, salt, and black pepper over the chicken and any vegetables that are not tucked underneath it.

Drizzle the olive oil and silan evenly over the chicken.

Spoon the garlic paste onto the chicken and rub it thoroughly over the entire surface, coating it well so it forms a rich garlic crust.

Drizzle the soy sauce over the chicken.

Pour the chicken stock around the sides of the pan, being careful not to wash off the garlic coating.

Seal the pan tightly with foil and bake for one and a half to two hours, depending on the size of the chicken, until tender and cooked through.

Remove the foil, baste the chicken generously with the pan juices, and return it to the oven uncovered for about five minutes to deepen the color and intensify the garlicky glaze.

Let rest briefly before serving, spooning the pan juices over the chicken and vegetables.