In a bowl, whisk together mirin, minced garlic, grated ginger, sesame oil, and soy sauce to create a flavorful marinade. Place your sirloin steak in a resealable plastic bag or dish and pour the marinade over it, ensuring the steak is fully coated. Let it marinate in the refrigerator for at least 30 minutes.

While the steak marinates, heat a large skillet over medium heat and add a drizzle of oil. Once hot, add sliced carrots, zucchini, and mushrooms. Sauté the vegetables for about 5-7 minutes until tender and slightly caramelized.

In the same skillet, add your pre-cooked fried rice, breaking it up as you stir it into the sautéed vegetables. Cook for about 3-4 minutes until heated through and slightly crispy.

Remove the marinated steak from the refrigerator and discard the marinade. Increase the skillet heat to high, searing the steak for about 3-4 minutes on each side until it reaches your desired doneness. Let the steak rest before slicing.

Assemble the bowls by placing a layer of fried rice in each bowl, topping with sautéed vegetables and arranging sliced steak on top. Drizzle with sesame oil and sprinkle with green onions or sesame seeds if desired.