Place 2 pounds boneless skinless chicken thighs in the bottom of the slow cooker insert.
In a medium bowl, whisk together ¼ cup light soy sauce, 2 tablespoons brown sugar, 1 tablespoon dark soy sauce, 1 tablespoon oyster sauce, 1 teaspoon ground ginger, 1 tablespoon minced garlic, 2 teaspoons sesame oil, and 2 teaspoons sriracha sauce.
Pour the sauce mixture evenly over the chicken thighs.
Cover and cook on LOW for 6–8 hours or on HIGH for 3–4 hours until the chicken is tender.
Remove the chicken from the slow cooker and shred it using two forks.
Return the shredded chicken to the slow cooker and stir it into the sauce.
Add 2 cups broccoli florets, 1 cup snow peas, 1 cup shredded carrots, 1 thinly sliced red bell pepper, and 1 drained 5-ounce can sliced water chestnuts to the slow cooker. Stir to combine.
Cover and cook on HIGH for 15 minutes until the vegetables are tender.
While the vegetables cook, boil 1 pound lo mein noodles according to the package directions. Drain well.
Add the cooked noodles to the slow cooker and stir everything together until evenly combined.
Garnish with sesame seeds and sliced green onions before serving, if desired.