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1
Make the spice blend: Combine 2 tablespoons kosher salt, the Old Bay, paprika, chili powder and garlic salt in a small bowl; set aside.
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2
Make the special sauce: Whisk the mayonnaise, ketchup, mustard, chipotle and 1 1/2 teaspoons of the prepared spice blend in a medium bowl until smooth; refrigerate until ready to use.
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3
Make the toppings: Heat the olive oil in a large skillet over medium-high heat.
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4
Add the mushrooms and cook, undisturbed, until browned on the bottom, about 2 minutes.
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5
Add the onion and season with salt and pepper.
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6
Reduce the heat to medium low and cook, stirring occasionally, until the vegetables are tender, 10 to 12 minutes.
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7
Remove from the heat and cover to keep warm.
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8
Grill the burgers: Preheat a grill to medium and brush the grates with vegetable oil.
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9
Form the beef into four 1-inch-thick patties and season generously with the spice blend.
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10
Grill until the bottoms are sizzling and browned, 5 to 6 minutes.
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11
Flip and cook 5 more minutes for medium.
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12
Top the burgers with the mushrooms and onions, then top each with a slice of cheese; let melt, about 1 more minute.
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13
Meanwhile, lightly toast the buns on the grill.
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14
Spread the special sauce on the buns.
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15
Sandwich with the burgers.
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16
Photograph by Steve Giralt
