Cook and shred the chicken: Heat the olive oil in a large pot over medium-high heat. Add the diced onion and sauté for 2-3 minutes, until softened. Place the chicken into the pot, sprinkle generously with taco seasoning, and add about 1 cup of water or broth. Cover and simmer for 15-20 minutes, until the chicken is fully cooked. Remove the chicken, shred it with two forks, and set aside. Leave the onions in the pot.

Make the cheesy Rotel sauce: In the same pot with the onions, melt the butter over medium heat. Whisk in the flour and cook for 1-2 minutes to form a roux. Slowly add the milk, whisking constantly until the mixture is smooth and lightly thickened. Stir in both cans of Rotel with their juices. Gradually add the shredded cheddar cheese, stirring until it is completely melted and the sauce is smooth. Mix in the sour cream, hot sauce, and an additional pinch of taco seasoning.

Combine and simmer: Return the shredded chicken to the pot with the cheesy Rotel sauce. Stir well to combine. Let the mixture simmer on low heat for 5-10 minutes, until it is creamy and thickened to your liking. Taste and adjust seasoning if needed.

Prepare taco shells: While the filling simmers, preheat your oven to 350°F (175°C). Warm the taco shells on a baking sheet for about 5 minutes, until crisp.

Assemble and serve: Fill each warm taco shell generously with the creamy chicken mixture. Garnish with fresh cilantro or green onions if desired. Serve hot