Roast the poblano chile on a gas flame or under an electric broiler. Roast chile well, turning so that it blackens on all sides.
Once the chile is completely blackened, place in a paper bag, then wrap the bag in a clean kitchen towel so that the chile steams and sweats. After 15 minutes, remove the chile from the bag and scrape off the charred skin.
Rinse under running water, then slice open lengthwise to remove the seeds and stem. Slice into strips, and set aside.
Preheat oven to 375°F.
In a 2 quart saucepan, whisk together Campbell’s® Condensed Cream of Mushroom Soup with one can of milk while heating over medium heat.
Bring to a simmer, and then turn off heat once the mixture is smooth and well combined. Stir in the cilantro and chile strips. Season to taste with salt and cracked black pepper.
Pour 1/3 of the hot soup into a 1 1/2 quart casserole dish. Place a layer of the sliced potatoes on the soup mixture, then a layer of sliced onion.
Continue layering the soup, potatoes and onions, topping the casserole with the last third of the soup mixture. Dot the top of the casserole with bits of the butter or margerine.
Cover with aluminum foil and bake for 1 hour.
Remove the aluminum foil and bake 15 minutes more or until potatoes are browned on top. Serve.