Heat toaster oven to 425° F. In a large bowl, whisk together the orange juice, tomato paste, garlic, cumin, cinnamon, 2 tablespoons of the oil, 1 teaspoon of the salt, and 1/2 teaspoon of the pepper. Add the pork and toss. Cover and refrigerate for at least 30 minutes and up to 8 hours. Meanwhile, in a bowl, combine the eggplant, onion, and the remaining oil, salt, and pepper. Place the vegetables on skewers, alternating eggplant and onion. Transfer the skewers to a foil-lined toaster-oven baking tray. Bake for 20 minutes. Turn the vegetables. Place the pork on skewers and add them to the tray. Bake until the vegetables are tender and the pork is cooked through, turning the pork and vegetables once, about 25 minutes. Meanwhile, wrap the bread in foil and place on top of the pork during the last 5 minutes. Transfer the skewers and bread to individual plates. Spoon the tzatziki (if using) on the side and sprinkle with the cucumber and mint.