Prep Vegetables & Sauce: Chop the 1/2 red onion, 1 carrot, 1 bell pepper, 1/2 head Napa cabbage, and sliced shiitake mushrooms. In a small bowl, whisk together 2 tbsp oyster sauce, 2 tbsp light soy sauce, 1 tsp dark soy sauce, 2 tbsp sugar, 2 tbsp Shaoxing rice wine, 1 tsp sesame oil, and 2 tbsp water until smooth.
Layer the Rice Cooker: Line the bottom of your rice cooker pot with the chopped Napa cabbage. Place the 2 frozen udon bricks directly on top of the cabbage layer. Add the chopped onion, carrot, bell pepper, and mushrooms over the noodles. Top the entire stack with the 1.5 lb of frozen thin-sliced beef.
Cook: Drizzle the mixed sauce evenly over the top layer of beef and vegetables. Close the rice cooker lid and set to cook for 40 minutes.
Toss and Serve: When the cooking cycle finishes, open the lid and use tongs to gently toss the beef, vegetables, and udon noodles together until evenly coated in the glossy sauce. Serve hot and enjoy your hands-off meal!