Preheat a grill or grill pan over medium.
In a large bowl, mix the beef, egg, garlic, cinnamon and nutmeg; season with 1 tsp. salt and 1/2 tsp. pepper. Form into 8 meatballs.
In another large bowl, toss the onion and eggplant with 1 tbsp. oil; season. Onto each of 8 skewers (soak wooden skewers in water 30 minutes before cooking), thread 1 piece onion, 1 piece eggplant, 1 meatball, another piece onion and another piece eggplant. Grill, turning, until the vegetables are tender and the beef is cooked through, 12 to 15 minutes.
In a medium bowl, toss the parsley, mint, feta, lemon juice and the remaining 1 tbsp. oil; season. Serve the kebabs on top of the herb salad.