Make the sauce: In a bowl, whisk together soy sauce, brown sugar, pineapple juice, rice vinegar, sesame oil, garlic, and ginger until the sugar dissolves.
Load the slow cooker: Place the meatballs into a 4- to 6-quart slow cooker. Pour the teriyaki sauce over the top and toss to coat.
Cook low and slow: Cover and cook on Low for 3 to 4 hours or on High for 1.5 to 2 hours, until the meatballs are heated through and the flavors meld.
Thicken the sauce: In a small bowl, whisk cornstarch with water to make a slurry.
Stir it into the slow cooker. Cover and cook for 15 to 20 minutes more, until the sauce is glossy and clings to the meatballs.
Adjust seasoning: Taste and add a pinch of red pepper flakes or a drizzle of sriracha if you like heat. If it’s too salty, add a splash more pineapple juice.
If it’s too sweet, add a bit more vinegar.
Garnish and serve: Sprinkle with sesame seeds and green onions. Serve over rice, with steamed broccoli or snap peas, or pile into slider buns.