To make the kebabs: Into a big bowl, throw the sirloin, parsley, yoghurt, za'atar, cumin, paprika, sumac, salt, some cracked pepper, garlic, egg and onion, and mix. Cover and refrigerate for 2 hours to marinate.
To make the garlic tahini: Preheat the oven to 160°C. Peel the papery skin off the garlic and slice off the top. Place on a piece of foil, drizzle with the olive oil, cover and bake for 1 hour. Throw the tender cloves into a food processor and add the tahini, yoghurt, lemon juice, mint, parsley, salt, sugar, cayenne and some cracked pepper and blend.
Preheat a grill pan on medium-high. Roll the beef mixture into fingers and skewer. Lightly coat the kebabs with the olive oil. Turn the grill pan down to medium and grill for 3 minutes per side.
Place 2 grilled kebabs inside a pita, spoon in some tahini sauce and top with pickled radishes.