For the tacos: Combine the soy sauce, garlic, sugar, sesame oil and 1/2 teaspoon pepper to make the marinade. Add the beef and mushrooms and marinate for at least 30 minutes. Cook in a skillet over medium-high heat, stirring, until cooked through.
For the salad: Combine the rice vinegar, sugar, fish sauce and garlic in a large bowl. Add the lettuce, green onions and red onion and toss everything together. Drizzle in the sesame oil and sprinkle in the sesame seeds. (As with many Korean dishes, you can easily adjust the recipe by adding more vinegar, sugar or salt to taste. Be sure to taste as you go!)
For the gochujang sauce: Mix together the gochujang, sesame oil, sugar, vinegar and 1 tablespoon water in a small bowl,, adding more water for a thinner consistency if desired.
For the taco assembly: Add the meat mixture to the tortillas and top with the salad. Drizzle as much or as little of the gochujang sauce as you desire. Enjoy!