Preheat your oven to 400°F (200°C). Stand the taco shells upright in a casserole dish or arrange them on a baking sheet so they will hold their fillings.

Warm a drizzle of oil in a skillet over medium-high heat. Add the diced onion and cook until it softens, about one minute.

Stir in the shredded chicken and the taco seasoning along with the amount of water called for on the seasoning packet. Reduce the heat and simmer until the liquid mostly evaporates and the chicken is heated through.

Fold the drained Rotel tomatoes into the chicken, then continue to simmer until the mixture thickens and excess tomato juices have reduced.

Spoon a layer of refried beans into the bottom of each taco shell, top with the chicken mixture, and finish by sprinkling with the Mexican cheese blend.

Bake the filled shells at 400°F for 5–7 minutes, until the cheese has melted and is bubbly.

Remove from the oven and serve right away with your favorite toppings such as sour cream, salsa, cilantro, or avocado.