In a large mixing bowl, combine 1 tablespoon of soy sauce, 1 tablespoon of rice vinegar, minced garlic, ginger powder, 1 teaspoon of sesame oil, ½ teaspoon black pepper, and ½ teaspoon salt (if using). Add the cubed steak and stir to coat thoroughly. Allow it to marinate for 30 minutes.

Heat 1 teaspoon each of sesame and olive oil in a large skillet over medium-high heat. Remove the steak from the marinade, discarding the marinade, and add the steak to the hot skillet. Cook for 3-4 minutes, or until well browned and a crust forms. Remove the steak from the skillet and set it aside.

In the same skillet, add 1 teaspoon of sesame oil and 1 tablespoon of butter. Toss in the zucchini, carrots, mushrooms, and onions. Season with garlic powder and the remaining 1 tablespoon of soy sauce. Stir-fry the vegetables for about 10 minutes, or until tender yet crisp. Transfer the vegetables to a plate.

In the same skillet, add the remaining 2 tablespoons of butter and 1 teaspoon of sesame oil over medium-high heat. Add the frozen fried rice and cook for 5-6 minutes, stirring occasionally. Toss the rice with the last tablespoon of soy sauce and cook for an additional 1-2 minutes, or until heated through.

Divide the fried rice evenly into four serving bowls. Top each bowl with the sautéed vegetables and browned steak. Drizzle with Yum Yum sauce and garnish with sliced green onions and sesame seeds. Serve immediately.