Marinate the Steak: In a medium bowl, whisk together soy sauce, sesame oil, garlic powder, ginger, and black pepper. Add the steak cubes and toss to coat. Let it marinate for at least 15 minutes, or up to an hour in the refrigerator.

Cook the Rice: While the steak marinates, ensure your rice is cooked and ready to go.

Prepare the Vegetables (Optional): If using vegetables, heat olive oil in a large skillet or wok over medium-high heat. Add the sliced bell pepper, onion, zucchini, and broccoli. Stir-fry until tender-crisp, about 5-7 minutes. Remove from the skillet and set aside.

Cook the Steak: In the same skillet or wok, heat a bit more oil (if needed) over medium-high heat. Add the marinated steak (reserve the marinade) and cook for 3-5 minutes, or until cooked to your desired level of doneness. Be careful not to overcrowd the pan; cook in batches if necessary.

Make the Hibachi Sauce: While the steak is cooking, in a small saucepan, combine soy sauce, rice vinegar, mirin, brown sugar, sesame oil, minced garlic, and minced ginger. Bring to a simmer over medium heat.

Thicken the Sauce: In a small bowl, whisk together the cornstarch and cold water. Pour the cornstarch slurry into the simmering sauce and stir continuously until the sauce thickens.

Combine and Serve: Add the cooked vegetables (if using) back to the skillet with the steak. Pour the thickened hibachi sauce over the steak and vegetables and stir to coat everything evenly.

Assemble the Bowls: Divide the cooked rice among bowls. Top with the steak and vegetable mixture.

Garnish and Enjoy: Sprinkle with sesame seeds and chopped green onions, if desired. Serve immediately.