Heat the oven to 425°F and oil a rimmed sheet. Mix breadcrumbs with meatball water, then gently fold in pork, egg, ginger, salt and garlic powder. Shape 24 meatballs and bake for 16 to 20 minutes until the centers reach 160°F.
Meanwhile, simmer broth, soy sauce, black vinegar, brown sugar, hoisin and pepper flakes in a saucepan for 2 minutes. Transfer hot sauce and hot meatballs to a 4-quart slow cooker.
Cover and cook on HIGH for 90 minutes. Fold in bell pepper and celery, whisk cornstarch with slurry water and stir it gently into the sauce.
Cover and cook on HIGH for 30 to 40 minutes more until the vegetables are tender-crisp, the sauce is glossy and the meatball centers reach 165°F.
Turn the cooker off and gently fold in sesame oil and peanuts. Serve about 3 meatballs per appetizer portion with sauce and vegetables.