Heat olive oil in a large pot over medium heat.

Add the diced onion and sauté until softened.

Stir in the ground beef and a generous amount of taco seasoning. Cook until browned and fully cooked, breaking it apart as it cooks.

Remove the beef and onions from the pot, leaving a little of the drippings behind.

In the same pot, melt the butter. Sprinkle in the flour and whisk for 1–2 minutes to form a light roux.

Slowly whisk in the milk until smooth and slightly thickened.

Stir in the Rotel (with juices) and gradually add the shredded cheddar. Continue stirring until the cheese melts into a smooth, velvety sauce.

Mix in the sour cream, hot sauce, and a pinch more taco seasoning. Adjust seasoning to taste.

Return the cooked beef and onions to the pot with the cheese sauce.

Stir everything together and let simmer on low for about 10 minutes, until the mixture is thick and creamy.

Arrange taco shells on a baking sheet and warm in a 350°F (175°C) oven for 5 minutes to crisp them up.

Fill each taco shell generously with the cheesy beef mixture, letting it spill over for that indulgent look.

Serve immediately, with Spanish rice on the side if desired.