First make the salsa. Preheat the broiler. While it is heating, prepare the corn. Holding each ear upright, cut off the kernels with a sharp knife. Heat a large heavy-based skillet or frying pan without any fat over high heat until almost smoking. Add the kernels and dry-roast until tender, smoky and dark, tossing continuously as they tend to stick, 4 to 5 minutes. Cook the peppers under the broiler, turning them until the skin chars and bursts, 7 to 10 minutes. Put them in a plastic bag and leave to sweat and cool so the skins loosen. Peel the peppers, discarding cores and seeds. Dice the flesh.
Heat 1 tablespoon of the olive oil in a large frying pan. Add the onion and saute until soft but not brown, 3 to 4 minutes. Take the pan from the heat and stir in the roasted corn, chopped tomatoes, sun-dried tomatoes, diced peppers, vinegar and remaining oil. Heat, stirring, until hot. Take from the heat and keep warm.
Rinse fish steaks and pat dry with paper towels. Brush with half the oil, season with salt and pepper and set on oiled grill pan or grill. Broil 3 to 4 minutes. Turn steaks over, brush with the remaining oil and grill until done to taste, 2 to 3 minutes longer.
Meanwhile, stir the cilantro and lime juice into the salsa, taste and adjust seasoning. Make a bed of salsa on 4 warmed plates and place a tuna steak on top of each. Serve at once.