First, prepare the pickled onions. Set 3 cups of water to boil. Thinly slice onions and place in a mesh sieve. Set aside. Add vinegar, salt, sugar, and garlic to a glass jar. Place lid on jar and shake to dissolve sugar and salt. Set aside.
Once water has reached boil point, hold sieve with onions over the sink and pour water over them VERY CAREFULLY. This will blanche the onions and take away some of their harsh bite. Once they've drained, add to the jar with the vinegar mixture. If some of the onions are still exposed, add a little water to submerge them. Shake to mix together and then place in fridge for at least 30 minutes. In the meantime, work on the rest of the salad.
For the salad:
Place lettuce in a shallow bowl. Top with corn, black beans, diced tomatoes, and mozzarella cheese. Once pickled onions have pickled for 30 minutes, add to salad, as well.
Add 1 tablespoons of BBQ sauce to the shredded chicken. Place on top of salad. drizzle salad with the rest of the BBQ sauce and ranch dressing. Serve immediately.