Make the spice blend: Combine 2 tablespoons kosher salt, the Old Bay, paprika, chili powder and garlic salt in a small bowl; set aside.
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.
Make the toppings: Heat the olive oil in a large skillet over medium-high heat. Add the mushrooms and cook, undisturbed, until browned on the bottom, about 2 minutes. Add the onion and season with salt and pepper. Reduce the heat to medium low and cook, stirring occasionally, until the vegetables are tender, 10 to 12 minutes. Remove from the heat and cover to keep warm.
Grill the burgers: Preheat a grill to medium and brush the grates with vegetable oil. Form the beef into four 1-inch-thick patties and season generously with the spice blend. Grill until the bottoms are sizzling and browned, 5 to 6 minutes. Flip and cook 5 more minutes for medium. Top the burgers with the mushrooms and onions, then top each with a slice of cheese; let melt, about 1 more minute. Meanwhile, lightly toast the buns on the grill.
Spread the special sauce on the buns. Sandwich with the burgers.
Photograph by Steve Giralt
Recipe courtesy of Baby's Badass Burgers for Food Network Magazine