1FOR THE KOREAN BEEF: In a small bowl, whisk together brown sugar, soy sauce, sesame oil, red pepper flakes, and ginger. Heat vegetable oil in a large skillet over medium-high heat. Add garlic and cook, stirring constantly, until fragrant, about 1 minute. Add ground beef and cook until browned, about 3–5 minutes, breaking up the meat as it cooks; drain excess fat.
2Stir in the soy sauce mixture until well combined. Simmer until heated through, about 2 minutes; set aside.
3FOR THE KIMCHI: Heat sesame oil in a small skillet over medium-high heat. Add kimchi and sugar, and cook, stirring constantly, until caramelized and heated through, about 3–5 minutes; set aside.
4FOR THE SRIRACHA MAYONNAISE: In a small bowl, whisk together mayonnaise, Sriracha, and lime juice; set aside.
5FOR THE TACOS: Serve ground beef mixture in tortillas, topped with kimchi, red onion, cilantro, and sesame seeds, drizzled with Sriracha mayonnaise.