To roast the corn, preheat the oven to 350 degrees. Place the corn cobs, husks intact, directly on the oven rack. Let cook for 35 minutes. After roasting, allow the corn to cool until it can be handled easily (5-10 minutes).
While the corn is cooling, dice the onion into small pieces and slice the tomatoes in half. Rinse the cilantro well, pull the leaves from the stalks, and roughly chop.
Once the corn is cool enough to handle, pull off the husks and corn silk. Either cut or break the base of the corn cob off so that there is a flat surface to stand the cob on. Stand the cob in a bowl and, using a small knife, cut straight down to remove the kernels. The kernels will fall right off into the bowl instead of all over the counter top. Continue to cut off the kernels from all six cobs.
Add the onion, tomato, and cilantro to the bowl with the corn. Stir to combine. In a small bowl, whisk together the olive oil, lemon juice, cumin, salt, and pepper. Pour over the salad and stir to coat. Enjoy immediately or refrigerate until ready to eat.