For the relish:
Mix all the ingredients in a small bowl and stir to combine. Season with salt if desired. Cover and refrigerate.
For the salad:
Shred the romaine lettuce and place in a bowl. Refrigerate. Mix the remaining ingredients in a small bowl, Cover and refrigerate until ready to serve then toss with the romaine.
Wash and dry the shrimp on paper towels. Place in a bowl and pour on the Korean BBQ sauce. Cover and keep in the refrigerator for 30-60 minutes. Heat a grill or grill pan to high heat. Place the shrimp on the grill and cook 2 minutes on each side. Remove the shrimp from the grill. Place the tortillas on the grill and heat until just warm.
Assemble the tacos with the shrimp on the bottom, topped with the romaine salad and onion cilantro relish. Garnish with hot sauce and avocado if desired.