Trim excess fat from beef and slice very thinly across the grain. Transfer beef to a container.
Place onion, pear, garlic, ginger, and sake in a blender or food processor. Blend until smooth and creamy. Add soy sauce, brown sugar, sesame oil, and black pepper. Pulse until marinade is well mixed.
Pour marinade over beef. Gently massage beef for a minute or two. Cover and refrigerate for at least 1 hour or overnight.
Mix vinegar, sugar, garlic, gochugaru, sesame oil, and salt together in a large bowl. Add cucumbers and green onions and mix gently by hand. Refrigerate for at least 30 minutes.
Remove beef from refrigerator about 20 minutes before cooking.
Heat canola oil in a cast iron pan over high heat. Cook beef in batches, one layer at a time, stirring until nicely browned, about 3 minutes. Avoid overcrowding beef, or it will end up steaming instead of browning. Transfer to a serving plate and sprinkle with sesame seeds.
Heat tortillas in another skillet, about 1 minute. Transfer to a serving platter and cover to keep warm. Serve beef bulgogi with the tortillas, cucumber salad, and avocado, green onions, and cilantro as garnishes. Drizzle gochujang over tacos, mixing it with a little water if too thick.
