Sprinkle the potato starch on the hot steamed rice. Mix to coat the grains but try to avoid crushing them. The starch will help the rice to firmly attach together.
Place a quarter of the rice in each mold (or in a plastic wrap if mold is unavailable). Cover and press the lid to shape the rice buns.
Coat a pan with sesame oil and add the rice buns. Lightly brown both sides. Brush one side with soy sauce and flip over. When the aroma of soy sauce grows stronger, remove the buns from the pan.
Season the beef slices with salt and pepper and add sake.
Heat vegetable oil in a pan. Add the sliced carrot, string bean pods, and onion. Stir fry the vegetables and distribute the oil evenly. When slightly softened, gather the vegetables to one side, making space for the meat. Add the oil again, and sautee the beef slices. You can also use pork or chicken instead. When the beef is almost cooked, reduce the heat to low.
Add half the teriyaki sauce to the vegetables and half to the meat. Distribute the sauce evenly, but stir fry the vegetables and the meat separately. Remove and spread the ingredients onto a tray to cool quickly. This will help keep the vegetables from getting soggy.
Now, let's make the rice burgers. Place the rice bun into a hamburger wrapper. Add the green leaf lettuce, stir-fried vegetables and beef slices. Sprinkle on the toasted white sesame seeds. Finally, cover the toppings with another rice bun.