In a large bowl, combine the first 5 ingredients. Add chicken; mix lightly but thoroughly. Shape into ¾-in. balls.
In a large skillet, heat oil over medium heat. Brown meatballs in batches; drain. Transfer to a 4- or 5-qt. slow cooker.
Add stock, tomatoes, onion, bok choy, carrots, red peppers, serrano peppers, garlic and salt. Cook, covered, on low 6-8 hours or until meatballs are cooked through and vegetables are tender.
If desired, stir in panko. Without stirring, drizzle beaten egg into slow cooker. Let stand until egg is set, 2-3 minutes.
