1. Preheat the grill or oven to 450°F.
2. Roast the corn on the grill or in the oven until it begins to turn golden, about 10 minutes.
3. Meanwhile, mix the lime juice, olive oil, vinegar, red onion, scallions, jalapeño and cilantro together in a bowl.
4. When the corn is cool enough to handle, cut the kernels from the cob and add them to the bowl. Pit, skin and chop the avocado and add that to the bowl.
5. Season with salt and pepper and set aside.
For the tuna:
1. Preheat the grill or broiler.
2. Mist the tuna steaks with olive oil and season them with salt and pepper.
3. Grill the tuna steaks on each side until just opaque, about 3 to 4 minutes per side. (Thinner steaks will take a little less time and thicker ones will take a little more.)
4. Serve the steaks topped with the relish.