It's also the perfect type of meat to cook in a crock pot or slow cooker. This article describes three methods of cooking rump roast: braised in the oven, cooked in a slow cooker, and marinated. 1  Prep time (Braised): 15 minutes.2  Cook time: 90 minutes. 3  Total time: 115 minutes.lace in the oven and roast for 1 1/2 hours, or 30 minutes per pound. 7. Test the roast to make sure it's done. Remove the lid or foil from the pan and insert a meat thermometer into the roast.