Preheat the oven to 400°F (200°C). Line a 9×13-inch baking dish with parchment paper or lightly grease it.
Place the 12 hard taco shells standing upright in the prepared baking dish so they can crisp without falling over. Bake for 5 minutes, then remove and set aside.
Heat 1 tablespoon olive oil in a large skillet over medium-high heat. Add 1/3 cup chopped red onion and cook about 3 minutes until softened.
Add 1 minced garlic clove and cook 1 minute until fragrant.
Add 3 cups shredded rotisserie chicken and 1/2 cup BBQ sauce to the skillet; stir until the chicken is evenly coated and heated through. Remove from heat and stir in 1 cup of the shredded Mexican blend cheese and 1 tablespoon chopped cilantro.
Spoon the chicken mixture evenly into the prepared taco shells. Sprinkle the remaining 1 cup shredded cheese over the tops.
Bake the filled tacos for 7 to 10 minutes, or until the cheese is melted and bubbly.
Remove from the oven and top each taco with finely shredded purple cabbage, corn salsa, sliced or diced avocado, and additional fresh cilantro as desired. Serve immediately.
