Preheat the oven to 400 degrees Fahrenheit. Spray a 9x13-inch baking dish with nonstick spray.
In a medium skillet, heat the olive oil over medium heat.
Add onions to skillet and cook for 2-3 minutes, or until translucent and fragrant.
Stir in the cooked, shredded chicken, taco seasoning, Rotel, water and salsa. Stir to combine fully. Reduce to simmer and allow to cook for 5-8 minutes.
Place the taco shells in the baking dish, standing up.
Bake the taco shells for 5 minutes by themselves to allow them to crisp up. Remove from the oven.
Fill the taco shells about ¾ full of the chicken mixture.
Sprinkle each shell generously with shredded cheese.
Bake for 7-10 minutes, or until the cheese is fully melted and the edges of the shells are browned.
Remove from the oven and top with your favorite items such as jalapenos, sour cream, shredded lettuce, cilantro, and salsa.