Cut the sirloin steak into small cubes. In a mixing bowl, combine the steak with 1 tablespoon of soy sauce, the hoisin sauce, sesame oil and garlic powder. Let it marinate for at least 10 minutes.
Heat a large skillet or wok over medium-high heat. Add one tablespoon of vegetable oil. Add the diced onion. Sauté until fragrant and the onions are translucent, about 2-3 minutes. Remove from the skillet and place in a large bowl.
In the same skillet, add the rest of the vegetable oil. Once hot, add the steak and cook until browned and cooked through about 4-5 minutes. Remove from the skillet and add to the onions.
Add the frozen mixed vegetables to the skillet. Cook for another 3-4 minutes until the vegetables are heated through.
Push the vegetables to one side of the skillet. Add a bit more oil if needed, and crack the egg into the empty side of the skillet. Scramble the egg until fully cooked, then mix it with the vegetables.
Add the cooked rice to the skillet, breaking up any clumps with your spatula. Return the steak and onions to the skillet. Pour in the remaining soy sauce and sprinkle with salt and black pepper. Stir well to combine everything and heat through about 3-5 minutes.
Remove from heat, garnish with chopped green onions, and serve hot.