Preheat oven to 400°F
In a large bowl, combine ground meat, chopped onion, egg, breadcrumbs, soy sauce, salt, and pepper. Mix well with your hands or a wooden spoon until just combined.
Use your hands to shape the mixture into meatballs. Place the meatballs on a baking sheet lined with parchment paper.
Bake the meatballs in the preheated oven for 15-20 minutes, or until cooked through.
While the meatballs are baking, prepare the teriyaki sauce. In a small saucepan, combine bourbon maple syrup, soy sauce, water, brown sugar, and garlic. Bring the mixture to a boil over medium heat, then reduce the heat to low and simmer for about 5 minutes, or until the sauce has thickened slightly.
Once the meatballs are done, remove them from the oven and place them in the teriyaki sauce. Let them simmer in the sauce for a few minutes to coat.