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1
Whisk mayonnaise and barbecue sauce in small bowl to blend.
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2
Cover and chill.
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3
Mix ground chuck with 1 teaspoon salt and 1/2 teaspoon pepper just to blend (do not overmix).
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4
Form mixture into six 1 1/2-inch-thick patties.
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5
Cover and let stand at room temperature 30 minutes.
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6
Prepare barbecue (medium-high heat).
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7
Run 1 metal skewer horizontally through center of each onion slice.
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8
Brush onion slices lightly with oil to coat.
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9
Rub chiles with oil.
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10
Transfer onions and chiles to barbecue.
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11
Grill onions until softened and browned, about 5 minutes per side.
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12
Remove skewers from onions.
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13
Chop onions; transfer to small bowl.
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14
Char chiles until blackened on all sides.
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15
Enclose chiles in paper bag 10 minutes.
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16
Peel, seed, and chop chiles.
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17
Mix into onions.
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18
Season onions and chiles to taste with salt and pepper.
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19
Sprinkle burgers with salt and pepper.
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20
Grill burgers until cooked to desired doneness, about 4 minutes per side for medium.
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21
Top each burger with 2 cheese slices during last minute of cooking.
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22
Place bottom halves of rolls on work surface.
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23
Divide chile-onion mixture among rolls.
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24
Place cheeseburgers atop chile-onion mixture.
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25
Spread 1 tablespoon mayonnaise-barbecue sauce over cheese, cover with bun tops, and serve, passing remaining sauce.
