Sauté: Heat olive oil in a large skillet over medium heat. Sauté the onion for 3–4 minutes until soft.
Brown: Add the ground beef and cook until fully browned, breaking it into small crumbles. Drain any excess grease. Stir in the taco seasoning and let it simmer for 2 minutes to meld.
The Roux: In a separate saucepan, melt the butter over medium heat. Whisk in the flour and cook for 1–2 minutes until it smells slightly nutty but doesn't brown.
Thicken: Slowly whisk in the milk to avoid lumps. Continue cooking and whisking until the sauce begins to thicken and coat the back of a spoon.
The Queso: Stir in the Rotel (with the juices). Gradually add the shredded cheddar, stirring constantly until the cheese is completely melted and the sauce is glossy.
Final Flavor: Fold in the sour cream, hot sauce, and a pinch of taco seasoning.
Assemble: Fill the warmed nacho cheese shells with the seasoned beef and generously ladle the hot Rotel sauce over the top.
Serve: Serve immediately while the sauce is at its creamiest, ideally with a side of Spanish rice.
