Grilled beef and mushroom burgers

Ingredients:

• 110 g sweet potato, peeled and diced (half a small tuber)
• 110 g cremini mushrooms, trimmed and quartered
• 450 g of ground beef with 20% fat content
• 1 large egg, lightly beaten
• 2 tsp cornstarch
• 1 bunch of green onions (about 6 small green onions)
• 1 medium poblano pepper, halved lengthwise, seeds removed
• 1 tbsp canola oil, plus extra for greasing grill grates
• 4 sesame seed hamburger buns, cut in half horizontally
• 4 slices pepper jack cheese (about 60 g)
• 1/3 cup mayonnaise
• Finely grated zest and juice of half a lime
• 1/4 cup pickled jalapeño rings
• 4 lettuce leaves

Preparation:

1. Preheat grill to medium-high heat.
2. Pulse the sweet potato in a food processor until evenly chopped, scraping down the sides of the bowl with a spatula as needed, about 10 pulses. Transfer to a large bowl. Pulse the mushrooms in a food processor until evenly chopped, scraping down the sides of the bowl with a spatula as needed, about 8 pulses. Add to the bowl with the sweet potato along with the ground beef, egg, cornstarch, 1 teaspoon salt, and black pepper to taste, then mix with your hands to distribute the ingredients evenly. Form the meat mixture into 4 patties, each about 10 cm in diameter. Season each side with a pinch of salt and freshly ground black pepper; set aside.
3. Toss the green onions and poblano peppers with vegetable oil. Grill the green onions until lightly charred, turning with tongs, 1 to 2 minutes. Transfer to a cutting board. Grill the poblano peppers, turning occasionally, until charred and tender, about 8 minutes. Transfer to a cutting board. Toast the hamburger buns on the grill until golden brown and marked, about 30 seconds. Transfer to a serving platter.
4. Lightly coat the grill grate with vegetable oil. Place the patties on the grill and cook, uncovered, until the bottom is well-marked, 3 to 4 minutes. Carefully flip the patties with a metal spatula and continue cooking until golden brown, another 2 minutes. Top each patty with a slice of cheese, then cover and cook until the cheese is melted and the patty is medium-rare, about 1 minute more. Transfer the patties to the bottom halves of the buns.
5. Thinly slice the green onions and place them in a small bowl. Add mayonnaise, lime zest and juice, a generous pinch of salt, and a few grinds of black pepper. Spread the mixture on the top halves of the buns. Thinly slice the poblano pepper and arrange it on top of the patties on the buns. Evenly arrange the pickled jalapeño and lettuce leaves on top. Top with the top halves of the buns.