Preheat oven to 425 degrees F.
Dice the onions into quarters. Slice the poblano pepper in half and remove the seeds and stem. Husk the tomatillos and discard the leaves. On the first parchment lined baking sheet, add one onion, poblano, two ears of corn and two cups of the whole heirloom tomatoes. Toss the veggies in 1-2 tablespoons of oil.
On the second parchment lined sheet pan, add the tomatillos and additional onion. Toss with remaining 1 tablespoon of oil. Using both oven racks roast the vegetables for 20-25 minutes, or until they are soft and have begun to char. Set aside to cool, about 5 minutes.
Set aside the pan with the roasted tomatillos. Remove the corn to a seperate plate and using a sharp knife slice the corn off the cob. From the first pan, finely chop the onion, poblano and heirloom tomatoes. Season the chopped vegetables with salt and garlic powder. Add the corn, fresh cilantro, shredded rotisserie chicken and toss. Season to taste with additional salt and garlic, if needed. Prepare the tomatillo salsa.