In a large bowl, whisk together tahini, garlic, cumin, cinnamon, cayenne, honey, lemon juice, 2 tablespoons olive oil, salt, and pepper. Whisk in 1/4 cup water, or slightly more to thin marinade.
Place chicken breasts in marinade and toss to coat evenly. Cover and place in fridge, 1 hour.
Preheat oven to 375 degrees. Place 2 tablespoons olive oil in a cast iron skillet over medium heat. Sauté onions until translucent in color, 4 minutes. Remove chicken breasts from marinade and place in skillet to brown, 2 minutes, on each side. Spoon 1/4 cup of marinade over the chicken and place in oven to roast until done, 15 to 20 minutes.
To serve, top with fresh chopped parsley and mint, and sprinkle with sesame seeds, if desired.