How to Make It
Bring vinegar, 1/3 cup water, 1 tablespoon brown sugar, pepper, and 2 garlic cloves to a boil in a saucepan over medium-high. Remove from heat; add cucumber slices. Cool to room temperature, stirring occasionally, about 30 minutes. Drain.
Meanwhile, stir together sirloin, panko, egg white, 1 1/2 teaspoons soy sauce, 1 1/2 teaspoons brown sugar, and remaining grated garlic clove in a bowl until well blended. Shape mixture into 16 meatballs, a scant 1 tablespoon each.
Heat a large skillet over medium. Coat pan with cooking spray. Add meatballs to pan; cook, stirring occasionally, until browned and cooked through, 5 to 7 minutes. Move meatballs to side of skillet. Add oil, remaining 1 1/2 tablespoons brown sugar, and remaining 1 1/2 tablespoons soy sauce to center of skillet; cook until sugar dissolves and sauce is glazy, about 1 minute. Stir to coat meatballs.
Layer 3 well-moistened paper towels on a microwave-safe plate. Place frozen buns on prepared plate; cover with 3 more well-moistened paper towels. Microwave at high until warmed, about 1 minute and 30 seconds, checking for doneness every 10 seconds after heating for 1 minute.
Divide cucumber slices, lettuce leaves, and meatballs evenly among buns.