Preheat the oven to 400 degrees F (200 degrees C). Pour some enchilada sauce into a baking dish, barely covering the bottom.
Lay out roast beef on a cutting board. Trim each slice to be the size of a taco. Cover with some Cheddar and Monterey Jack cheese. Sprinkle some onion over cheese and roll up beef slices. Arrange rolls in the baking pan. Moisten the top with enchilada sauce. Sprinkle remaining cheese, onion, and olives over enchilada sauce. Cover baking dish with aluminum foil.
Bake in the preheated oven until heated through, about 15 minutes. Uncover and continue baking until cheeses brown, about 7 minutes more. Serve with sour cream.
