Slice beef very thin against the grain. Place in a zip top bag.
In a mixing bowl, whisk together sesame oil, garlic, ginger, soy sauce, brown sugar, water, and Gochujang. Pour over the beef. Close bag, massage to incorporate, and place in fridge to marinate for at least 4 hours or up to 24 hours.
Just before cooking beef, make slaw by whisking vinegar, sugar, oil, salt and pepper in a medium sized bowl. Add cabbage and toss to combine. Cover and place in fridge.
Next, make the sauce by whisking sour cream, mayo, garlic powder, salt, pepper, sriracha, and lime juice together. Place in fridge until ready to eat.
Heat a large, non-stick skillet to medium-high heat. Add meat (with marinade) and sauté for 3 minutes. Stir then continue cooking for an additional 3-5 minutes.
To assemble, warm tortillas then place meat, slaw, and sauce in them. Garnish with cilantro and avocado.