Heat a gas grill to medium high or prepare a medium-hot charcoal fire. Remove the thick, outer leaves of the corn husk. Peel back the inner leaves, being careful to leave them attached to the cob. Remove as much of the corn silk as possible and fold the inner leaves back over the corn. It’s fine if some of the kernels show through the gaps inthe leaves.
Trim the ends of the onion, peel it, and cut it into 1/2-inch slices. Thread the slices on skewers, brush liberally with olive oil, and season with salt.
Once the fire is hot, put the corn and skewered onion slices on the grate and cover the grill. Grill the onion slices, flipping once so that they brown on both sides, for 8 to 10 min. total. Wrap the slices in foil and set aside; they’ll continue to cook and soften a bit before they cool. Grill the corn for 10 to 15 min., flipping often until the husk is very charred and the kernels feel cooked when pressed with a fingernail; some of the kernels will be browned. Set aside to cool slightly before cutting the kernels off the cobs. Coarsely chop the cooled onion slices into 1/2-inch dice.
Heat a small skillet over medium heat. Toast the cumin seeds in the hot, dry pan, shaking frequently, until fragrant, about 20 seconds. Seal the seeds in a plastic bag and coarsely crush them with a rolling pin or heavy pan (or use a coffee or spice grinder or mortar and pestle).
Combine the cumin seeds, lime juice, jalapeño, and 1 tsp. salt in a small bowl. Gradually whisk in the 5 Tbs. of oil to blend.
Lightly season the tomato chunks with salt. Transfer to a medium bowl and add the corn, onion, black beans, and cilantro. Drizzle the vinaigrette over the salad and toss gently. Fold in the avocado. Season to taste with salt and pepper. Garnish with the scallion and serve immediately.