In a saucepan over medium-high heat, whisk together water, soy sauce, brown sugar, cornstarch, rice wine vinegar, and grated ginger until smooth.
Continue whisking constantly for 5 to 6 minutes until the sauce thickens to a gravy-like consistency, then remove from heat.
Line a baking sheet with parchment paper.
In a large mixing bowl, combine ground beef, panko, onion, eggs, milk, salt, pepper, and grated ginger until fully mixed.
Use a 1 ½ tablespoon scoop to portion and roll the mixture into meatballs, placing them on the prepared baking sheet.
Heat olive oil in a nonstick skillet over medium-high heat.
Brown the meatballs on all sides using tongs, about 7 to 8 minutes.
Reduce heat to medium and spoon the teriyaki sauce over the browned meatballs.
Simmer the meatballs in sauce for 8 to 10 minutes, turning occasionally to prevent burning and until internal temperature reaches 160°F.
Garnish with sliced green onions and toasted sesame seeds before serving.