Roast the poblano chiles individually over the flame of your stovetop, using metal tongs to hold them, until the skins are totally charred. (If you don't have a gas stove, you can use an outdoor grill OR place them on a baking sheet directly under your oven's broiler.) Place the blackened chiles in a large resealable plastic bag and seal the bag to allow the chiles to sweat for a good 20 minutes.
Use a knife to scrape off most of the blackened skins. Slice the chiles in half lengthwise, scrape out the seeds and membranes and chop into small dice.
Add the butter to a very large pot and saute the onions, jalapenos and garlic over medium heat. Add the milk, cream, chili powder and some salt and pepper and heat. Add the processed cheese and stir to melt. Then add the Cheddar, Monterey Jack, tomatoes and diced poblanos. Slowly melt and keep warm.
This is a large-quantity recipe for feeding a crowd, but it can be scaled down quite easily.
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