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1
Roast the chilies over an open flame or close up under a preheated broiler, turning them regularly, until evenly blackened and blistered, about 5 minutes for an open flame, 10 minutes for the broiler.
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2
Place in a bowl, cover with a kitchen towel and cool until handle-able.
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3
Rub off the blackened skin, pull out the seedpods and scrape out the seeds.
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4
Rinse briefly to remove any stray seeds and bits of blackened skin.
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5
Cut into 1/4-inch strips.
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6
Set a large (10-inch) skillet over medium heat.
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7
Add the chorizo and cook, stirring to break up the clumps, until it is beginning to brown and is cooked through, about 10 minutes.
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8
Scrape onto a plate lined with paper towels and let cool.
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9
Return the skillet to medium heat, measure in the oil and add the onion.
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10
Cook, stirring regularly, until it is soft and beginning to caramelize, about 7 minutes.
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11
Stir in the garlic and poblano and cook for 2 minutes.
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12
Taste, season with salt, usually about 1/2 teaspoon, scrape the rajas into a bowl and cover to keep warm.
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13
In a large bowl, combine the ground beef, the cooled chorizo and the chipotles.
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14
Mix thoroughly but lightly (to keep from turning out an overly compact texture).
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15
Divide into 4 portions, lightly pressing them into patties the size of your buns.
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16
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 quite hot, then bank the coals to one side.
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17
Lay the hamburger patties on the hottest side of the grill and cook until the grill grate has seared beautiful marks on one side, about 2 minutes, then flip and cook until the hamburgers are a little less done than you like (usually a couple of minutes longer for rare to medium-rare).
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18
Move the burgers to the cooler side of the grill.
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19
Lay one piece of cheese on each burger, top with a portion of the warm rajas and then another piece of cheese.
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20
Close the lid and continue cooking until the cheese has melted, about 1 minute.
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21
Remove from the grill and place on the toasted buns.
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22
Serve immediately.
