Follow these step-by-step instructions to make Korean Beef Bulgogi that is sure to impress your family and friends:

Step 1: Prepare the Marinade

1. In a large bowl, combine the soy sauce, brown sugar, grated Asian pear or apple, minced garlic, chopped green onions, sesame oil, sesame seeds, and black pepper.
2. Mix the ingredients thoroughly until the sugar is dissolved and the marinade is well-blended.

Step 2: Marinate the Beef

1. Add the thinly sliced beef into the marinade, ensuring every slice is well-coated.
2. Cover the bowl with plastic wrap or transfer the mixture to a resealable plastic bag.
3. Marinate in the refrigerator for at least 30 minutes, preferably 2 hours or overnight for richer flavor.

Step 3: Prepare the Vegetables

1. While the beef is marinating, prepare the vegetables.
2. Julienne the carrot, slice the onion thinly, and prepare the mushrooms if using.

Step 4: Cook the Bulgogi

1. Heat a large skillet or grill pan over medium-high heat.
2. Add a small amount of oil to the pan if necessary to prevent sticking.
3. Once the pan is hot, add the marinated beef slices in a single layer.
4. Cook for 2-3 minutes on each side until the beef is browned and cooked through.
5. Depending on the size of your pan, you may need to cook the beef in batches.

Step 5: Add Vegetables

1. Once the beef is cooked, add the prepared vegetables to the pan.
2. Stir-fry for an additional 2-3 minutes until the vegetables are tender-crisp.

Step 6: Final Touches

1. Sprinkle additional sesame seeds and chopped green onions over the finished dish before serving.