Tenderize the meat & marinate: Pound each chicken thigh inside a ziplock bag using a meat mallet until ½ inch thick. Repeat for all thighs and set aside.
Prepare the Marinade: In a mortar & pestle, combine paprika, coriander seeds, peppercorns, dried rosemary, red chili flakes, and sea salt. Grind to a semi-coarse powder, then add chopped garlic and continue to grind until well blended. Incorporate the olive oil and lemon juice to create the marinade.
Marinate the chicken: Coat the chicken thighs evenly with the marinade, wrap in cling film, and refrigerate overnight, or at least 2 hours if short on time.
Assemble the rotisserie: Stick a bamboo skewer vertically through a large onion half to create a base and pole.
Skewer and cook: Skewer the marinated chicken thighs individually on the bamboo skewer, stacking them to form a mound. Preheat oven to 375°F (190°C) and bake on a cookie tray for 1 hour, or until internal temperature reaches 165°F (74°C). Let cool for 15 minutes, then slice the meat thinly to serve with tzatziki sauce.