In a mixing bowl, combine ground beef, soy sauce, brown sugar, sesame oil, minced garlic, grated ginger, and red pepper flakes. Mix well until the beef is evenly coated.
Heat a skillet over medium-high heat. Add the marinated beef and cook until browned and cooked through, about 5-7 minutes, breaking it apart with a spatula as it cooks.
While the beef is cooking, prepare the toppings. Wash and slice the avocado, shred the carrots, and slice the cucumber and green onions.
Warm the tortillas in a separate pan or by wrapping them in foil and placing them in a preheated oven for a few minutes.
Assemble the tacos by placing a generous amount of the cooked beef on each tortilla. Top with shredded carrots, cucumber slices, green onions, and avocado.
Drizzle gochujang sauce over the tacos for a spicy kick and garnish with fresh cilantro if desired.
Serve immediately and enjoy your Korean Beef Tacos!