Step 1Heat the broiler.
Step 2On a baking sheet, place the peppers. Place the sheet directly under the broiler and broil, turning a few times with tongs, until blackened on all sides, 6 to 8 minutes total. Remove from the broiler and place in a bowl, covering it with plastic wrap to steam. When the peppers are cool enough to handle, after 15 to 20 minutes of steaming, remove the skin, chop the flesh and set aside. Alternatively, you can char the poblano peppers over an open flame on the stove, turning with tongs until all sides are charred, before steaming.
Step 3Adjust the oven temperature to 425 degrees and bring a pot of water to a boil.
Step 4In a 2-inch-deep cast-iron skillet, melt the butter over medium-high heat. Add the onion, green onions and roasted peppers and cook until the onion is softened, about 5 minutes. Remove the skillet from the heat and top the onion and pepper mixture with the cheeses. Add the milk and stir to combine.
Step 5Place the skillet in a roasting pan or baking dish that is deeper than the skillet, and set the roasting pan in the oven. Add enough hot water to the roasting pan or baking dish to come 1 inch up the side of the skillet, making sure the water doesn't come close enough to spill into the skillet. Bake until the cheese is melted and gooey, 20 to 25 minutes. Remove from the oven, sprinkle with cilantro, and serve immediately with tortilla chips.