In a large mixing bowl, combine the panko bread crumbs and milk and let rest for a few minutes. If the mixture seems too dry add a few extra drops of milk to moisten the bread crumbs.
Add sirloin, pork, onion, soy sauce, salt and pepper. Knead the meat until it becomes sticky and binds together. Divide into 4 equal parts (a scale comes in handy).
Lightly dab your hands with sesame oil. Using your palms, roll each part of the meat into a ball, then pat the ball flat, shifting it from hand to hand to form a ½-inch-thick patty. Make a shallow indentation across the center of the patty to keep it from puffing while it grills.
Grill the burgers, flipping twice, until browned and cooked through with no pink in the middle, about 10 minutes (160 degrees). Serve on buns, topped with spicy ketchup and shredded iceberg lettuce.
Recipe by My Lilikoi Kitchen at https://www.mylilikoikitchen.com/2016/11/26/japanese-burgers/