Preheat the oven to 425°F.
Marinate the chicken – Pat your chicken dry. In a large bowl, combine the chicken and sliced onions with the marinade ingredients – olive oil, lemon juice, garlic, worcestershire sauce, dijon mustard, dried oregano, dried thyme, smoked paprika, kosher salt, and black pepper. Cover and let it sit at room temp for at least 30 minutes.
Bake the potatoes – Lay the potatoes out on a large parchment paper lined baking sheet in a single layer. Drizzle them with olive oil, kosher salt, and black pepper. Bake in the preheated oven for 10 minutes.
Add the chicken – Remove the baking sheet from the oven, give the potatoes a toss, and push them to one side. Add the marinated chicken and onions right on the baking sheet with all the juices, and return it all to the oven. Bake for another 10 minutes.
Season the leeks – In a small bowl, toss leeks, lemon zest, olive oil, and kosher salt together.
Add the leeks & finish baking – Remove the baking sheet from the oven once more, scatter the leek mixture over top of the potatoes. Return to the oven and bake for 15 more minutes until the leeks are tender and golden brown, and the chicken is cooked through.
Make the yogurt sauce – In a medium bowl, combine the greek yogurt, grated garlic, and kosher salt.
Serve with a swipe of the yogurt sauce on the plate and top fresh parsley.