special equipment: 8 wooden skewers that have been soaked in water or 8 metal skewers.
1. Mix the butter, brown sugar, chili powder, cumin, orange zest, 1 tablespoon salt, and 1 teaspoon pepper together in a bowl. Add the beef, toss to coat evenly. Cover and refrigerate for 15 minutes or up to 1 hour.
2. Preheat a grill over high heat. Thread the beef, onions, and peppers onto 8 long skewers leaving 1/4-inch space between the meat and vegetables. Lightly brush the grill grates with oil. Arrange the skewers on the hot grill and grill, turning until the meat is medium-rare, about 2 to 3 minutes on each of the 4 sides. Transfer the kebabs to the platter and serve.