Slice the Beef: Take 1.5 pounds of beef sirloin or ribeye and place it in the freezer for about 30 minutes. This makes it easier to slice. After chilling, cut the beef into thin strips, about 1/8 inch thick. Set aside.
Make the Marinade: In a large bowl, mix 1/4 cup of soy sauce, 2 tablespoons of brown sugar, 2 tablespoons of sesame oil, 3 minced garlic cloves, 1 teaspoon of grated ginger, and 1/4 teaspoon of black pepper. Stir until the sugar dissolves.
Add Ingredients: Peel and slice 1 medium onion and 1 large carrot into thin pieces. Add these to the marinade. Toss in the sliced beef, and ensure each piece is coated with the marinade. Cover the bowl and refrigerate for at least 1 hour or up to overnight for best results.
Cook the Bulgogi: Heat a large skillet over medium-high heat. Once hot, add the marinated beef and vegetables in a single layer. Cook for about 5-7 minutes, stirring occasionally, until the beef is browned and cooked through.
Serve: Transfer the Korean beef bulgogi to a serving plate. Enjoy it with steamed rice and fresh lettuce leaves for wrapping. Optionally, add some kimchi or garnish with chopped green onions for extra flavor.