Step 1: In a medium bowl, combine the thinly sliced beef with 1 tablespoon soy sauce, sesame oil, 1/2 teaspoon black pepper, and cornstarch. Mix well and let marinate for at least 10 minutes.
Step 2: While the beef marinates, prepare the sauce. In a small bowl, whisk together 2 tablespoons soy sauce, oyster sauce, mirin, sugar, 1 teaspoon black pepper, minced garlic, grated ginger, and water or beef broth. Set aside.
Step 3: Heat 1 tablespoon vegetable oil in a large skillet or wok over high heat until shimmering. Add the marinated beef in a single layer (cook in batches if necessary to avoid overcrowding). Sear for 1-2 minutes per side until browned and just cooked through. Remove beef from the pan and set aside.
Step 4: In the same skillet, add the frozen corn kernels and optional sliced red bell pepper. Sauté for 2-3 minutes until bright and slightly tender. Remove from pan and set aside.
Step 5: Add the 2 tablespoons of butter to the hot skillet. Once melted and sizzling, pour in the prepared sauce mixture. Bring to a gentle simmer, stirring constantly, until the sauce slightly thickens (about 1 minute).
Step 6: Return the cooked beef and corn/bell pepper to the skillet with the sauce. Toss to coat evenly.
Step 7: To assemble the bowls, place hot cooked rice in individual serving bowls. Top generously with the sizzling beef and vegetable mixture. Serve immediately.