1. Preheat a grill rack or grill basket to medium high on the grill. Place the whole peppers and onion slices on the grill rack. Grill for about 15-20 minutes, flipping the peppers from side to side until the skin is blackened. Grill and flip the onion slices until they are slightly charred and slightly cooked.
2. Remove onions and peppers and let cool about 15 minutes.
3. While the peppers and onions cook, cut the watermelon into wedge-shaped quarters. Cut each quarter into 1-inch-thick slices. Grill the slices for 2 to 3 minutes per side, or until grill marks appear and the melon is slightly softened. You may need to press the watermelon flesh down with a spatula to get good contact with the grill to mark the flesh. Remove and let cool.
4. When peppers are cool enough to handle, peel off the charred skin. Hold peppers over a large bowl to catch any juices as you remove the cores. Remove any seeds that fall in, and discard. Chop peeled peppers into 1/2-inch pieces and add to the bowl. Chop the onions and add to the bowl. Remove the rind from the melon, chop the grilled flesh into 1/2-inch pieces, and add.
5. Mix the peppers, onions, and melon together. Add the oil, lime juice, mint, hot sauce, salt, and pepper. If you want a spicier salsa add more hot sauce. Chill and serve within 3 to 4 hours.
Source: fresh Magazine May, June 2010