I created this delicious, indulgent recipe when I was asked to participate in one of those culinary challenges that have become part of every chef’s life nowadays: A hamburger challenge. —Rick Bayless
Roast the chiles over an open flame or close up under a preheated broiler, turning them regularly, until evenly blackened and blistered, about 5 minutes for open flame, 10 minutes for the broiler. Place in a bowl, cover with a kitchen towel and cool until handleable. Rub off the blackened skin, pull out the seed pod and scrape out the seeds. Rinse briefly to remove any stray seeds and bits of blackened skin. Cut into 1/4-inch strips.
Set a large (10-inch) skillet over medium heat. Add the chorizo and cook, stirring to break up the clumps, until the chorizo is beginning to brown and is cooked through, about 10 minutes. Scrape on to a plate lined with paper towels and let cool.
Return the skillet to medium heat, measure in the oil and add the onion. Cook, stirring regularly until it is soft and beginning to caramelize, about 7 minutes. Stir in the garlic and poblano and cook for 2 minutes. Taste and season with salt, usually about 1/2 teaspoon. Scrape the rajas into a bowl and cover to keep warm.
In a large bowl, combine the ground beef, the cooled chorizo and the chipotles. Mix thoroughly but lightly (to keep from turning out an overly compact texture). Divide into 4 portions, lightly pressing them into patties the size of your buns.
Heat a gas grill to medium-high on one side, medium on the other; or light a charcoal fire and let it burn until the charcoal is covered with white ash (and still quite hot), then bank the coals to one side.
Lay the hamburger patties on the hottest side of the grill and cook until the grill grates have seared beautiful marks on one side (about 2 minutes if your grill is quite hot), then flip and cook until the hamburger is a little less done than you like (usually a couple of minutes longer for rare to medium rare). Move the burgers to the cooler side of the grill.
Lay one piece of cheese on top of each burger, top with a portion of the warm rajas and then another piece of cheese. Close the lid and continue cooking until the cheese has melted about 1 minute. Remove from the grill and place on a toasted bun. Serve immediately.