Juice the lemon into a mixing bowl. Add the plain yogurt, ground cumin, paprika, ras el hanout, sumac, granulated garlic, and salt.

Add the boneless chicken thighs and coat them completely in the marinade. Cover and refrigerate for 90 minutes.

Peel the potatoes if desired and cut them into large rough chunks. Place them in a large pot of cold water.

Bring the potatoes to a boil and par-cook them for 10 to 15 minutes. Drain the potatoes thoroughly in a colander.

Preheat the oven to 200°C / 390°F. Halve the red onions and place the skewers upright through the onion halves on the baking sheet.

Thread the marinated chicken thighs onto the upright skewers to form a stacked shawarma tower. Arrange the par-cooked potatoes around the chicken.

Drizzle the potatoes with olive oil and season them with a little paprika and granulated garlic. Roast for 30 minutes.

Turn the chicken tower and stir the potatoes. Roast for another 15 minutes, until the chicken is cooked through and the potatoes are crisp.

Slice the cooked chicken thinly from the skewers. Slice the tomatoes and remaining red onion, and shred the lettuce.

Spread tahini and hummus inside each pita bread. Fill with chicken, roasted potatoes, lettuce, tomatoes, and red onion, then serve immediately.