1) Prepare a hot barbecue outside or preheat a griddle pan to high heat, and preheat the oven to 190C/Gas 5.
2) Toss the raw chicken in a bowl with the rub, a few pieces at a time, until all of the chicken is well coated. Cook the chicken on well-oiled barbecue or griddle pan for about 2 minutes on each side, until nice marks are formed. If the chicken pieces are very thick, finish cooking for 5 to 10 minutes more in the preheated oven. Allow the chicken to cool for a few minutes and then pull apart into nice shreds. Toss the chicken with barbecue sauce so that it is well coated but not overly wet. Keep warm in a warm saute pan until ready to use.
3) Heat the olive oil in large saute pan over medium-high heat. Stir in the garlic with a wooden spoon. After 30 seconds, add the spinach and turn off the heat. Toss the spinach in hot oil and, when wilted completely, season with salt. Set aside.
4) Spread butter evenly over one side of each pita. In a large saute pan, cook each pita bread over medium-high heat, buttered side down (work in batches if necessary). Top each pita with 45g of cheese, a quarter of the spinach and a quarter of the chicken. Fold each pita over and continue cooking until golden brown on each side.
Cut each pita in half crosswise, transfer to platter and serve.
Smoky barbecue rub:
1) Combine all of the ingredients, using a whisk, wooden spoon or hands. Use hands to break up the lumps of brown sugar.
1) Heat the oil in a 1-litre saucepan over medium heat and sweat the onions until translucent, about 5 to 10 minutes.
2) Add the garlic and sweat for 1 more minute. Add the dry spices and barbecue rub; stir. Cook until the spices become fragrant, about 1 minute more. Add the remaining ingredients and simmer slowly for 20 minutes. Using an immersion blender, puree the sauce until smooth.