Honestly, although I made these with venison, pork is more traditional. And you can basically use any ground meat here. But, the ground meat needs to be fatty, at least 15 percent fat (20 percent is better), and it should be ground relatively fine. A coarser ground meatball will still taste great, but it won't be as refined. Your choice.
Mix all the meatball ingredients together in a bowl. Mix all the sauce ingredients in another bowl.
Form meatballs anywhere from the size of a walnut to the size of golf ball. For best results, set the meatballs on a baking sheet and put it into the fridge for an hour to firm up. But you can cook the meatballs straight away if you'd like.
Cook the meatballs. You can deep fry them at 360°F for about 5 minutes, or you can poach them in simmering water for about the same amount of time (they're ready when they float); or you can bake the meatballs at 400°F for about 20 minutes.
Glaze the meatballs. Whisk the sauce together so the starch doesn't stick to the bottom of the bowl, and pour it into a large saute pan. Bring it to a boil and add the cooked meatballs. Roll them around in the hot sauce to glaze for 30 seconds or so. Move the meatballs to a serving plate and sprinkle sesame seeds over them. Serve hot as an appetizer or with rice.
Notes
I don't like a super sweet sauce, so I only use 1 tablespoon of sugar; mirin is also sweet, so that's enough for me. Feel free to increase the amount of sugar if you like.