Rinse tuna; pat dry. Brush tuna with olive oil; sprinkle with salt and pepper. For charcoal grill, grill tuna on greased rack of uncovered grill directly over medium coals, 8 to 12 minutes or until fish flakes easily when tested with fork and center is slightly pink, turning once. (For gas grill, preheat grill. Reduce heat to medium. Place tuna on greased rack over heat. Cover; grill as above.)
Meanwhile, cook potato slices in boiling salted water 5 to 8 minutes or until tender. Drain; set aside Cut corn from cob .
In small bowl combine cilantro, green onions, and jalapeno; cover and chill. Prepare Lime Dressing; set aside. Break tuna into 3/4-inch pieces.
Place tuna evenly on bottom of 3-quart rectangular baking dish. Drizzle with a 1/3 of the Lime Dressing. Add potatoes; drizzle with another 1/3 of the dressing. Add corn and drizzle with remaining dressing. Sprinkle lightly with salt and pepper. Cover and chill in refrigerator 2 to 3 hours
To serve, top with cilantro mixture and sweet pepper. Sprinkle with chili powder; pass lime wedges. Makes 6 servings.
Finely shred 2 teaspoons of peel from 3 limes. Squeeze enough lime juice to equal 1/2 cup. In small bowl whisk together lime peel, lime juice, olive oil, garlic, coriander, cumin, salt, and freshly ground pepper.