Heat a gas grill to medium high or prepare a mediumhot charcoal fire.
In a small bowl, mix half of the garlic, 2 tsp. of the olive oil, the cumin, lime juice, 1/2 tsp. salt, 1/2 tsp. pepper, and a large pinch of cayenne. Spread an equal amount of the rub on both sides of the tuna and let it sit for 5 minutes.
Trim the ends of each squash and cut lengthwise into 1/2-inch-thick slices. Core and seed the red peppers and cut each into 4 pieces. Pile the squash and red peppers into a bowl. Drizzle with the remaining 3 Tbs. olive oil and sprinkle liberally with salt and pepper and another large pinch of cayenne. Toss to coat evenly.
Set the tuna steaks on the hottest part of the grill and fan the peppers and squash around them. Turning the vegetables occasionally, grill the tuna 3 minutes per side for medium rare or until cooked to the doneness you like. Transfer the tuna to a clean plate and tent loosely with foil. Continue grilling and turning the vegetables until they’re just tender, another 2 to 4 minutes, and transfer to a cutting board. Scrape the pepper skins off with a paring knife, if you like. Coarsely chop the vegetables and put them back into the same bowl in which they were tossed. Sprinkle the balsamic vinegar, half the cilantro, and the remaining garlic over the vegetables and toss to coat evenly. Taste for salt and pepper. Cut the tuna steaks into four portions.