Let's cook.
In a medium bowl, combine the soy sauce, brown sugar, sesame oil, minced garlic, and grated ginger to create the marinade.
Add the thinly sliced beef sirloin to the marinade, ensuring all pieces are coated. Let it marinate for at least 10 minutes at room temperature, or up to 30 minutes in the refrigerator.
Heat a large skillet or wok over medium-high heat. Once hot, add the marinated beef in a single layer. Cook for 2-3 minutes per side, until browned and cooked through. You may need to cook the beef in batches to avoid overcrowding the pan.
Remove the cooked beef from the skillet and set aside. Add the julienned carrot to the same skillet and stir-fry for 2 minutes until slightly tender-crisp.
Add the baby spinach to the skillet with the carrots and cook for 1-2 minutes, until wilted.
Return the cooked beef to the skillet with the vegetables and toss everything together to combine.
Divide the cooked white rice among four serving bowls. Top each bowl generously with the beef and vegetable mixture. Serve immediately.
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