Make the pickled onions an hour or two ahead of the pizzas. Place the thinly sliced onions in a mason jar or other glass jar with a lid. Combine the rest of the ingredients in a pyrex pitcher and microwave for two minutes. Remove and stir until sugar and salt are dissolved. Carefully pour over onions and cool for 30 minutes. At this point they are ready to use, but I always prefer them pickled a little more.
Pizzas: Drizzle a layer of olive oil over a rimmed cookie sheet. Wipe both sides of the tortilla in the oil enough to just cover. Top each tortilla with 1-2 tablespoons of barbeque sauce and 1/2 cup of pork or chicken. Layer 1/2 cups of cheese on top of all. Bake at 400 for about 10 minutes, cheese should be melted and starting to brown and edges just crisping.
Garnish with cilantro and pickled red onion.