Preheat oven to 200°C / 400°F and prepare a nonstick baking tray.
Put celery, carrot and onion in a food processor and process until very fine. Alternatively, you can just finely chop them using a knife.
In a large mixing bowl, add all ingredients for the meatballs and mix well until combined. If the mixture looks too wet, add more panko crumbs, 1 tbsp at a time.
Take 1 large spoonful of mixture and roll into a ball (around the size of a golf ball) using your hands. Transfer to baking tray. Repeat until all the mixture is used up.
Bake meatballs for 15 minutes or until cooked through.
While the meatballs are baking, make your glaze by combining all ingredients for the glaze in a microwave-safe bowl. Microwave for 2-3 minutes in 30 second intervals, stirring every pause, until the mixture thickens, and let it cool. It will continue to thicken as it cools down.
Remove meatballs from oven. Pour teriyaki glaze over them and toss gently to coat evenly.
Serve immediately with steamed rice or vegetables. Enjoy!