Heat gas grill to medium-high or prepare charcoal grill with medium-hot coals. Lightly coat grill rack with oil or cooking spray.
In a small bowl, stir together the cocoa-chile blend, cumin and garlic salt. Set aside.
Cut potatoes in half lengthwise and then crosswise into 1-inch slices. Brush cut sides with 1 tablespoon oil and sprinkle each with 1/4 teaspoon of the cocoa-chile mixture. Place 2 halves in foil and sprinkle about 3 tablespoons of the onion over each. Wrap tightly and place on the grill. Close lid and grill for 35 to 45 minutes or until tender.
Brush steak with remaining oil. Rub remaining chile mixture over both sides of steak. Grill for 2 to 3 minutes per side for medium-rare.
Slice the steak and serve with sweet potatoes, and lime wedges and chopped cilantro, if desired.
