Preheat the oven to 400°F (200°C). Line a 9x13 baking dish with foil or parchment paper.
Heat about 1 tablespoon of olive oil in a large skillet over medium heat.
Add the minced onion and cook 3–4 minutes, stirring, until softened and lightly golden.
Stir in the shredded cooked chicken, black beans (if using), salsa or diced tomatoes, and taco seasoning. Cook 3–4 minutes until heated through, stirring occasionally.
Arrange the hard taco shells upright in the prepared baking dish.
Spoon a few tablespoons of the chicken mixture into each taco shell, then sprinkle each with shredded cheese.
Bake 12–15 minutes, until the cheese is melted and shells are heated through.
Remove from the oven, garnish with cilantro, and serve.