Marinate the beef: In a bowl, mix soy sauce, sesame oil, brown sugar, garlic, ginger, rice vinegar, black pepper, green onion, and onion. Add beef and marinate for at least 30 minutes (or up to 4 hours in the fridge).

Cook the beef: Heat a pan over medium-high heat. Cook the marinated beef for 4–6 minutes until cooked through and slightly caramelized. Set aside to cool.

Prepare sushi rice: Mix rice vinegar, sugar, and salt in a small bowl. Microwave for 15–20 seconds to dissolve. Stir into the cooked sushi rice and let it cool slightly.

Assemble the rolls: Lay a sheet of nori on a bamboo sushi mat, shiny side down. Spread a thin layer of sushi rice over 2/3 of the sheet, leaving the top edge uncovered.

Add fillings: Layer a few strips of bulgogi beef, cucumber, carrot, and avocado across the center of the rice.

Roll it up: Using the mat, gently roll the sushi away from you, pressing tightly. Seal the edge with a little water.

Slice and serve: Cut into 6–8 pieces with a sharp knife. Sprinkle with sesame seeds and serve with soy sauce or spicy mayo.