In a mixing bowl, combine the fresh orange juice, lime juice, olive oil, minced garlic, brown sugar, ground cumin, salt, paprika, cayenne pepper, black pepper, dried parsley, and ground fennel. Whisk until well combined.
Cut the beef rump steak into 1-inch cubes and add them to the marinade. Make sure each piece is well coated. Cover the bowl and refrigerate for at least 1 hour, preferably 4 hours or overnight for maximum flavor.
Once the beef is marinated, take your skewers and thread the beef cubes and chunks of red onion onto them. Alternate between the beef and onion for a colorful presentation.
Preheat your grill to medium-high heat. If you're using a grill pan, heat it over medium-high heat on your stovetop.
Place the assembled kebabs on the grill. Cook for about 10-12 minutes, turning occasionally, until the beef is nicely charred and cooked to your desired level of doneness.
Remove the kebabs from the grill and let them rest for a few minutes. Serve with fresh lemon wedges on the side for an extra zing.