Combine Greek yogurt, fresh lemon juice, minced garlic, tomato paste, and olive oil in a bowl
Add all spices (cumin, chili powder, paprika, turmeric, onion powder, salt, and black pepper) to the yogurt mixture and stir well
Add chicken thighs to the marinade and coat thoroughly
Arrange marinated chicken on a sheet pan with sliced red onion, lemon slices, and whole garlic head
Drizzle vegetables with oil and season with salt and pepper
Roast in oven until chicken is cooked through and vegetables are tender
Remove roasted garlic cloves from the head and set aside for sauce
Prepare garlic sauce by combining mayonnaise, Greek yogurt, roasted garlic cloves, lemon juice, finely chopped parsley, salt, and black pepper
Serve chicken and vegetables with garlic sauce and garnish with fresh parsley
