Brown the Beef: In a large skillet, heat the olive oil over medium heat. Add the diced onion and cook until translucent, about 3 minutes. Add the ground beef and taco seasoning, breaking the meat apart as it cooks until fully browned. Drain any excess grease from the pan and set the beef aside.

Make the Cheese Sauce: In the same skillet, melt the butter over medium heat. Whisk in the flour and cook for 1-2 minutes to form a roux. Slowly pour in the milk, whisking constantly until the mixture is smooth and slightly thickened.

Stir in both cans of Rotel (with their juices). Reduce the heat to low and gradually add the shredded cheddar cheese, stirring until it is completely melted and the sauce is creamy.

Finish the Sauce: Mix in the sour cream and optional hot sauce. Season with salt and pepper to taste.

Combine and Simmer: Add the cooked beef back into the skillet with the cheese sauce. Stir everything together until the beef is well coated. Let the mixture simmer over low heat for 8-10 minutes, until it is thick, glossy, and scoopable.

Warm the Tortillas: Heat the flour tortillas in a dry skillet or microwave until they are soft and pliable.

Assemble the Tacos: Spoon a generous amount of the cheesy beef filling into each warm tortilla. Garnish with diced tomatoes and chopped cilantro. Serve immediately.