Not all blended beef and vegetable burgers are sturdy — or flavorful enough — to stand up to the grill. Our version is packed with meaty mushrooms and sweet potato, and is crowned with bold toppings like pepper jack, charred poblanos, pickled jalapenos and a grilled-scallion mayo. A little egg and cornstarch help the patty stay together on its journey on and off the grates, resulting in a juicy burger everyone will love.
Pulse the sweet potato in a food processor until evenly and finely chopped, scraping down the sides of the bowl with a rubber spatula as needed, about 10 pulses. Transfer to a large bowl. Pulse the mushrooms in the food processor until evenly and finely chopped, scraping down the sides of the bowl with a rubber spatula as needed, about 8 pulses. Add to the sweet potato along with the ground beef, egg, cornstarch, 1 teaspoon salt and a few grinds of pepper, then mix with your hands until combined. Form the meat mixture into 4 patties, each about 4 inches in diameter. Season each side with a good pinch of salt and a few good grinds of pepper; set aside.
Toss the scallions and poblano with the oil. Grill the scallions until lightly charred, turning halfway through with grilling tongs, 1 to 2 minutes. Transfer to a cutting board. Grill the poblano, turning occasionally, until charred all over and tender, about 8 minutes. Transfer to cutting board. Grill the hamburger buns until lightly toasted and have some grill marks, about 30 seconds. Transfer to a serving platter.
Brush the grill grates with some oil. Place the patties on the grill and cook, undisturbed (and with the grill uncovered), until the first side has developed nice grill marks, 3 1/2 to 4 minutes. Carefully flip the burgers with a metal spatula and continue to cook until well browned all over, about 2 minutes more. Top each burger with 1 slice of cheese, then cover and cook until the cheese has melted and the burger has reached medium doneness, about 1 minute more. Transfer to the bottom bun halves.
Thinly slice the scallions and place in a small bowl. Stir in the mayonnaise, lime zest and juice, a good pinch of salt and a few grinds of pepper. Spread the mixture on the top bun halves. Thinly sliced the poblano, then divide among the burgers. Top evenly with the pickled jalapenos and lettuce. Sandwich with the top bun halves.
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