Pat chicken dry with paper towel and cube it into large chunks.
In a big bowl, mix the marinade ingredients: Greek yogurt, tomato paste, garlic, lemon juice, and all those glorious spices.
Toss in the chicken until every piece is coated. Let it marinate as long as you can resist.
Preheat the oven to 425°F (220°C). Slice the top off a garlic head, drizzle with olive oil and salt, then wrap it up in foil and place in a dish/tray.
Pop the wrapped garlic in the oven for 45–60 minutes.
Spread the marinated chicken chunks on a large casserole dish or sheet pan.
Nestle in the red onion chunks, then drizzle 4 tbsp olive oil over everything.
Bake for 30–40 minutes until the chicken is browned, crisp at the edges, and cooked through.
Remove the tray from the oven and baste the chicken with the pan juices.
Squeeze the roasted garlic into a bowl, mash it then mix with Greek yogurt, mayo, lemon juice, pepper, and parsley. Taste and adjust.
Recommended plating: A bed of fluffy rice, topped with a handful of the shawarma chicken and red onion, a squeeze of fresh lemon, a showering of chopped parsley, and a drizzle of those beautiful pan juices.