Heat oven to 500°. Line a baking pan with foil.
Combine the flour, Cajun seasoning, and pepper. Cut several slashes over the roast. Rub the flour mixture all over the roast. Put the roast in the foil-lined pan and put in the oven. Reduce heat to 475° and roast for 20 to 25 minutes, or about 7 minutes per pound.
Turn off oven but don't open the door. Leave the roast in the oven for 2 1/2 hours. If you use a meat thermometer -- you have to be able to see it through the oven window -- you can get a more precise temperature. The roast should reach 145° for medium rare; 160° for medium.
Serves 6 to 8
Calories per serving, based on 6 ounces of roast beef per person: 373