Preheat the oven to 400 degrees. Place the ground pork or chicken in a large bowl. To the bowl add the remaining meatball ingredients. Mix until just evenly combined.
Cover a baking sheet with foil. With wet hands roll the meatball mixture into small balls, about one tablespoon each. Don't squeeze too tightly or the meatballs will become tough. The yield should be about 30 meatballs. Place the meatballs on the baking sheet as you roll them. Bake in the preheated oven until cook through golden brown, about 30-35 minutes.
In the meantime, begin cooking the rice according to the package directions: Place the rice and water in a medium pot, cover, and bring to a rapid boil over high heat. As soon as it reaches a full boil, turn the heat down to the lowest setting. Allow to simmer on low for 20 minutes, then turn the heat off and let the rice rest, undisturbed, until the rest of the meal is complete. The rice will continue to steam after the heat is turned off, so don't remove the lid.
While the meatballs are in the oven, prepare the teriyaki glaze. In a medium pot combine the soy sauce, water, brown sugar, rice vinegar, sesame oil and ginger. Heat over a low flame until the brown sugar is dissolved, just a few minutes.
Dissolve the cornstarch in just enough water to make it pourable. Pour it into the pot with the glaze ingredients. Turn the heat up to medium high, stir and cook until thickened. It should thicken as soon as it begins to simmer. Once thickened, add the sesame seeds.
When the meatballs are done, transfer them from the baking sheet into the pot with the glaze. Stir to coat. Fluff the rice with a fork. Build the bowls by placing one cup of rice in a bowl, add five meatballs and a couple of spoons of extra teriyaki glaze, then sprinkle a few sliced green onions over top.