Whisk together mayonnaise, lime juice, sriracha, crushed red pepper, and salt in a medium bowl. Add cabbage and onion; toss well to coat. Let stand at room temperature while beef cooks.

Whisk together stock, brown sugar, soy sauce, gochujang, and cornstarch in a small bowl; set aside. Heat oil in a large skillet over medium-high. Crumble beef into skillet, and stir in garlic. Cook, stirring often to break up beef into small pieces, until browned, 4 to 5 minutes. Stir in scallions, and cook, stirring often, until scallions are softened, about 2 minutes. Add reserved stock mixture. Bring to a simmer over medium-high; cook, stirring often, until sauce is thickened and glossy, 2 to 3 minutes. Remove from heat.

Spoon about 1/2 cup beef mixture onto each bottom bun. Top each with about 1/2 cup cabbage mixture, and cover with top buns.