Preheat your oven to 375°F (190°C) to prepare for baking the tacos.
In a large skillet, heat the olive oil over medium heat. Add the diced onion and sauté until translucent and fragrant, about 5 minutes.
Add the shredded cooked chicken, taco seasoning, drained diced tomatoes, and diced green chiles to the skillet. Stir and cook for 3-4 minutes until heated through.
Arrange the hard taco shells upright in a baking dish. Spoon a layer of refried beans into each shell, then add the chicken mixture on top.
Sprinkle shredded Mexican blend cheese generously over the filled taco shells.
Bake in the preheated oven for 15-20 minutes until cheese is melted and bubbly and taco shells are crispy.
Remove from oven, let cool for a minute, then top with sliced jalapeños, sour cream, salsa, shredded lettuce, and chopped fresh cilantro. Serve immediately.