1. About 10 minutes ahead, preheat gas grill or about ½ hour ahead, light charcoal briquettes.
2. In a small bowl, mix together onions, garlic, lemon juice, the oil, soy sauce, paprika, the salt, pepper, and cayenne.
3. Add chicken; toss to coat. Let stand 10 minutes or up to 24 hours, covered and refrigerated.
4. Thread 1 tender and 1 or 2 pieces of bell pepper onto each skewer. Discard marinade.
5. Grill, covered, over hot fire/coals until no longer pink in center, about 3 minutes per side. Brush lightly with melted butter, if desired. Serve immediately.