In a bowl, marinade beef with garlic, honey, soy sauce, oyster sauce, mirin, onion powder, and black pepper. Let it marinate while you prepare the rice.
Rinse 2 cups of rice in the rice cooker pot. Add the appropriate amount of water back to the pot.
Layer the sliced onions over the rice. Place the marinated beef on top, then add the corn.
Start the rice cooker and let it cook until done.
Once the rice is ready, open the cooker and add butter and chopped green onions. Gently mix everything together.
Scoop into bowls and enjoy warm.