You need to start a flat-cut rump roast at least 4 or 5 hours before you want to serve it. A flat-cut rump roast, also sold under the name "bottom round flat," is one of three cuts taken from the rump, along with the eye of round and beef round bottom. Flat-cut rump roasts are not the toughest of the three, but they are not tender enough to cook without a helping hand from mechanical tenderizing. You can still marinate for flavor, but it doesn't tenderize rump roast effectively.
Tenderize the roast using a meat mallet or blade tenderizer, then marinate it for 3 or 4 hours, if desired. Flat-cut rump roasts need to be aggressively tenderized both before and during cooking, so a quick pounding with the knobby side of a meat mallet or a few punctures from a blade tenderizer helps a lot. Use an acidic marinade to add flavor and help make the cut tender as well.
Take the roast out of the refrigerator and let it reach room temperature. If you didn't marinate the roast, season it now.
Heat the oven to 225 degrees Fahrenheit. Sear the roast on all sides in a few tablespoons of oil heated in a braising pan, then transfer the meat to a plate.
Saute mirepoix -- or battuto, which is mirepoix with minced garlic and flat-leaf parsley -- until soft in the braising pan.
Deglaze the pan with wine or stock, then place the roast in it. Add enough stock and wine to cover the meat by at least half and cover the pan with a lid or heavy-duty foil. Place the pan in the oven.
Braise the roast for about an hour and turn it over in the pan. Check the liquid every hour and add water as needed to keep it about halfway up the sides of the meat.
Return the pan to the oven and braise it until the meat separates easily with a fork or about 2 to 3 hours after you turn it over. Take the pan out when the roast tenderizes to your liking. You can also check the tenderness after you braise the roast for roughly 30 to 45 minutes per pound.
Transfer the roast to a plate and strain the liquid in the pan through a sieve and into a saucepan or pot.