Cook the ground beef. Heat the olive oil in a large pot over medium heat. Add the diced onion and sauté until softened. Add the ground beef and season generously with taco seasoning. Cook until browned, breaking it into crumbles. Once cooked, remove the beef and onions from the pot, leaving a bit of drippings behind.

Make the cheesy Rotel sauce. In the same pot, melt the butter over medium heat. Whisk in the flour and cook for 1–2 minutes. Slowly pour in the milk, whisking continuously until smooth and slightly thickened. Stir in both cans of Rotel. Gradually add the shredded cheddar cheese, stirring until fully melted and creamy. Mix in the sour cream, hot sauce, and a bit more taco seasoning. Adjust to taste.

Combine the beef and cheese sauce. Return the cooked beef and onions to the pot with the cheese sauce. Stir well to combine. Let the mixture simmer over low heat for about 10 minutes, until thick and creamy.

Warm the taco shells. Preheat your oven to 350°F. Arrange the taco shells on a baking sheet and warm them for about 5 minutes.

Assemble and serve. Spoon the cheesy beef mixture generously into each warmed taco shell. Serve hot with a side of Spanish rice if desired.