Heat grill to medium-high. Brush the corn and the cut sides of the onion with 1 tablespoon of the olive oil. Season with ¼ teaspoon each salt and pepper. Lay the corn flat on the grill and place the onion halves on the grill cut sides down. Grill, turning occasionally, until the vegetables are tender and slightly charred, 8-10 minutes. Transfer to a cutting board to cool.

Once cool enough to handle, dice the onion into small pieces. Place the corn vertical on the cutting board and, holding it upright firmly, use a sharp knife to cut the corn off the cob.

In a large bowl, combine the jalapeños, grape tomatoes, garlic, ½ teaspoon salt, and ¼ teaspoon pepper. Add the corn, onion, lime zest, lime juice, remaining 1 tablespoon of olive oil, diced avocado, and cilantro. Toss to combine. Serve immediately.