Step 1

Working the meat as little as possible, roll chicken into 4 meatballs (4 oz. each); press down slightly to make tops flat.

Step 2

Heat oil in a large cast-iron skillet over medium-high. Working in 2 batches, toast hamburger buns, cut sides down, until golden, about 1 minute. Transfer to plates.

Step 3

Add half of the bacon to same skillet and cook until browned and crisp, about 2 minutes per side. Place 4 pieces on top of 2 of the bottom bun.

Step 4

Season 2 meatballs generously with salt and pepper. Place them far apart in skillet. Using a spatula, smash meatballs down as thin as you can (use another kitchen tool to drive the spatula down toward the skillet to help flatten the meat against it). Top each with about a quarter of the onion and the jalapeño; press into meat. Cook patties, untouched, until browned underneath, about 2 minutes. Flip over and cook until patties are cooked through and onion is golden, about 1 minute. Transfer patties to bottom buns with bacon. Repeat process with remaining bacon, chicken, onion, and jalapeño to make 2 more burgers.

Step 5

Close up hamburgers and serve with ketchup alongside.