Preheat your grill to 300-350°F for indirect cooking.
Trim the root end and tip off each onion, remove the outer skin, then make one cut from the outside toward the center, stopping about halfway through.
Carefully separate the onion into individual layers to create shells.
In a large bowl, combine the ground beef, diced jalapeño, diced Roma tomato, chopped parsley, and seasoning. Mix until evenly incorporated, being careful not to overmix.
Stuff each onion layer with the beef mixture, gently pressing the meat in until filled.
Thread three stuffed onions onto each skewer, leaving a little space between each one for even cooking.
Grill at 300-350°F, turning every 15-20 minutes, until the onions are tender and the beef reaches an internal temperature of 165°F.
During the last 5 minutes of cooking, combine the melted butter and Cajun seasoning. Brush generously over the stuffed onions for a rich, flavorful finish.
Remove from the grill, let rest for 5 minutes, and serve with your favorite dipping sauce.
