Preheat the broiler. Place the poblano chiles on a small rimmed baking sheet. Broil for 10 to 15 minutes, turning the chiles occasionally, until charred on all sides. Remove from the oven and transfer to a resealable bag. Let rest for about 20 minutes, until cool enough to handle. Peel the chiles, remove the seeds, and slice into thin strips.
Preheat the oven to 400°F and spray a large oval baking dish with nonstick cooking spray. Toss together the chiles, potatoes, onions, and 1 cup of cheese and transfer to the prepared baking dish. Whisk together the heavy cream and milk in a small bowl and season with salt and pepper, to taste. Pour evenly over the potato mixture. Sprinkle the remaining cup of cheese over the top. Bake for 45 minutes to 1 hour, until the potatoes are tender. Remove from the oven and let rest for 10 minutes before serving.