Carefully cut kernels from corn cobs. You should have about 2 cups. Seed and peel mangoes. Cut each mango into 12 slices. Peel the outer layer off the onion. Cut onion into 6 wedges, leaving the root end intact so wedges stay together.
For a charcoal grill, place a 9- to 10-inch cast iron skillet on the grill rack directly over medium-hot coals. Heat skillet for 2 to 3 minutes. Add corn kernels and the 1 teaspoon oil. Cook and stir constantly for 6 to 8 minutes or until corn kernels are golden brown. Remove skillet from the grill; transfer corn kernels to six individual serving bowls. Set aside. (For a gas grill, preheat grill. Reduce heat to medium-hot. Place cast iron skillet on the grill rack directly over the heat for 2 to 3 minutes. Add corn. Cook and stir as directed above.)
In a medium bowl combine mango slices and onion wedges; drizzle with 1 tablespoon of the remaining oil. Toss gently to coat. For a charcoal grill, place mango slices and onion wedges on the grill rack directly over medium-hot coals. Grill mango for 3 to 5 minutes or until lightly browned, turning to brown evenly. Grill onion wedges for 5 to 6 minutes or until golden brown and crisp-tender, turning to brown evenly. Transfer mango slices and onion wedges to the medium bowl. (For a gas grill, reduce heat to medium. Place mango slices and onion wedges on grill rack directly over heat. Cover and grill as above.)
For dressing, in a small bowl combine lemon juice, sugar, curry powder, salt and pepper. Slowly whisk in the remaining 3 tablespoons oil until well combined. Add 1 tablespoon of the dressing to the mango mixture; toss gently to coat. Drizzle arugula with about half of the remaining dressing; toss to coat. Add arugula to corn; add mango and onion wedges. Drizzle with any remaining dressing. Makes 6 servings.