Preheat the oven to 400°F (200°C). Lightly spray a 9x13-inch baking dish with nonstick spray and stand the 10 hard taco shells upright in the dish so they fit snugly.
Heat 1 tablespoon olive oil in a medium skillet over medium heat.
Add 1/2 cup diced onion to the skillet and cook 2–3 minutes, until translucent and fragrant.
Stir in 1/2 pound shredded cooked chicken, 1 ounce taco seasoning, 14.5 ounces fully drained diced tomatoes, and 4.5 ounces fully drained diced green chiles. Reduce heat to low and simmer 5–8 minutes, stirring occasionally, until heated through and combined.
Place the baking dish with the taco shells in the oven and bake the empty shells 5 minutes to crisp them. Remove the dish from the oven.
Spoon about 1 tablespoon refried beans into the bottom of each shell, then fill each shell with the chicken mixture, packing it almost to the top.
Sprinkle each filled shell generously with shredded Mexican blend cheese.
Bake the assembled tacos 7–10 minutes, until the cheese is melted and the shell edges are lightly browned.
Remove from the oven and top with sliced jalapeños, sour cream, shredded lettuce, chopped cilantro, and salsa as desired.
