Heat sesame oil in a large skillet over medium heat. Add pork; cook and stir until pork is no longer pink, about 5 to 7 minutes. Add scallions, ginger, lemongrass, and garlic powder; cook and stir until fragrant, 2 to 3 minutes.
Reduce heat to low. Stir in soy sauce and ketchup. Cook until flavors combine and mixture thickens slightly, 3 to 5 minutes. Season with salt and pepper.
Combine cabbage, wasabi mayonnaise, rice vinegar, and lime juice in a large bowl; toss to mix slaw.
Split each bread roll and fill with pork mixture. Top with slaw before serving.