Combine all marinade ingredients in a blender and purée into a paste. Coat the chicken in the paste and marinate overnight or for at least 4 hours.
Preheat the oven to 350°F.
Remove the chicken from the marinade, discard marinade and season chicken with salt and pepper. Roast on a baking sheet until cooked through, about 22-28 minutes. Slice the chicken into ½ inch pieces on the bias.
Place the rice, 4 cups of water and a pinch of salt in a saucepan. Bring to a simmer, cover, and cook until the grains are tender, about 20 minutes. Remove from heat. Stir in the spinach, cover, and steam for 5 minutes, until wilted. Stir in the herbs, feta, lemon juice and oil. Season with salt and pepper.
Combine all of the tzatziki ingredients in a small bowl.
To serve, divide the spanakopita rice and spiced chicken between plates. Top with tzatziki and serve with a lemon wedge.