In bowl mix together your hoisin sauce, soy sauce, mirin, garlic, brown sugar, and green onions.
Place meatballs in a slow cooker. Pour sauce over top, and stir. Set to low for 4-6 hours.
Cover and cook on high 1-2 hours or low 2-4 hours.
Once cooked sprinkle with diced onions and sesame seeds.
Notes
To cook on the stove top:Place meatballs into a large skillet with a lid, pour sauce over top. Set to medium, and bring to a slow boil, reduce heat to low and let simmer 20 minute until meatballs are tender and the sauce has started to thicken.Remove lid, and reduce further, or you can stir in a slurry made with 2 Tbsp of Cornstarch and water, and cook until sauce thickens to desired consistency.