Heat oil in a large skillet over medium-high heat.
1 tbsp vegetable oil
Season chicken with salt and pepper all over and then add to skillet and cook until lightly browned, about 10 to 15 minutes.
Salt and pepper, 1½ lbs chicken breasts
Place cooked chicken into slow cooker.
Add remaining ingredients, except cornstarch and water (don't include the noodles/rice); stir to combine.
3 medium carrots, 6 green onions, 3 celery stalks, 1 tbsp fresh ginger, 3 cloves garlic, 8 oz. bean sprouts, ⅓ cup soy sauce, ¼ tsp red pepper flakes, 1 8 oz. can water chestnuts, 1 cup chicken stock, 1 tbsp granulated sugar
Cover and cook on HIGH for 3 to 4 hours, or on LOW for 6 to 10 hours.
About 10 minutes before serving, mix together the cornstarch with the water in a small bowl or measuring cup. Add to the slow cooker and stir. Stir occasionally until thickened, about 10 minutes.
¼ cup cornstarch, ½ cup cold water
Meanwhile, cook the noodles according to the package instructions and then mix them into the chicken mixture. Stir until fully mixed.
Chow mein noodles
Keep on warm until ready to serve, or, serve at once. Garnish with extra sliced scallions, red pepper flakes, and toasted sesame seeds.