Green Falafel Smash Burgers

Ingredients

• 1 medium red onion, ½ thinly sliced, ½ coarsely chopped
• 3 Tbsp. white wine vinegar
• 1½ tsp. Diamond Crystal or 1 tsp. Morton kosher salt, plus more
• 2 garlic cloves, finely grated
• 1 jalapeño, coarsely chopped
• 1 cup coarsely chopped tender herbs (such as parsley and/or cilantro)
• 1 tsp. ground cumin
• Freshly ground pepper
• 2 15.5-oz. cans chickpeas, rinsed
• 1 large egg, beaten to blend
• 2 oz. white cheddar, coarsely grated
• 1 cup panko
• ¾ cup mayonnaise
• 1 (heaping) Tbsp. harissa paste
• 4 hamburger buns
• 3 Tbsp. (or more) extra-virgin olive oil
• Lettuce leaves and sliced cucumbers (for serving)

Steps

1. Cut out 11 small pieces of parchment paper (about 5×5"); set aside. Toss ½ medium red onion, thinly sliced, 3 Tbsp. white wine vinegar, and a pinch of kosher salt in a small bowl. Let sit until ready to serve.
2. Place ½ medium red onion, coarsely chopped, 2 garlic cloves, finely grated, 1 jalapeño, coarsely chopped, 1 cup coarsely chopped tender herbs (such as parsley and/or cilantro), 1 tsp. ground cumin, and 1½ tsp. Diamond Crystal or 1 tsp. Morton kosher salt in a food processor, season with freshly ground pepper, and pulse until finely chopped. Scrape down sides, then add one 15.5-oz. can chickpeas, rinsed; pulse until finely chopped and homogeneous in size. Add remaining one 15.5-oz. can chickpeas, rinsed, and pulse a few times to coarsely chop, leaving some larger pieces. Transfer chickpea mixture to a large bowl; mix in 1 large egg, beaten to blend, 2 oz. white cheddar, coarsely grated, and 1 cup panko. Let sit 10 minutes (mixture should hold together when some is squeezed in your hand).
3. Meanwhile, mix ¾ cup mayonnaise and 1 (heaping) Tbsp. harissa paste in a small bowl; set aside.
4. Scoop out ⅓ cup chickpea mixture. Roll and pack into a tight ball with your hands. Place ball on a reserved parchment square on a surface. Place another square on top and press with your palm to form a ⅓"–½"-thick patty. Pat in any craggy bits to create smooth outer edges. Remove top piece of parchment. Repeat with remaining chickpea mixture to make 9 more patties, reusing top piece of parchment to smash all patties and leaving each patty on its own bottom piece of parchment.
5. Heat a dry large cast-iron skillet over medium-high. Working in batches, cook 4 hamburger buns, cut sides down, until toasted, 1–2 minutes per batch. Transfer to plates.
6. Heat 3 Tbsp. extra-virgin olive oil in same pan over medium-high. Working in 3 batches and using a slotted spoon to remove any stray bits of patties and adding more oil as needed between batches, lay patties in skillet, then peel away parchment. Cook until deeply browned, about 3 minutes per side. Transfer to a baking sheet.
7. Build burgers with buns, lettuce leaves, sliced cucumbers, reserved harissa mayo, reserved pickled onion, and 2 patties each. (Wrap remaining 2 patties and reserve in refrigerator for another use. You can reheat in a dry hot skillet until warmed through, 1–2 minutes per side.) Do Ahead: Patties can be formed 1 day ahead. Wrap tightly and chill.