Heat gas or charcoal grill. Cut 12 (10x10-inch) sheets of heavy-duty foil. In 1-quart saucepan, melt butter over medium heat. Cook onion in butter until onion is tender, stirring frequently. Stir in 1/3 cup cilantro and bell pepper; cook 3 minutes, stirring constantly. Stir in lime juice and garlic salt.