Corned beef responds best to long slow baking with some moisture. It retains its pink color when cooking, so the best way to judge doneness is with a thermometer. Your corned beef roast will be done when the internal temperature reaches 145 degrees Fahrenheit, measured with a thermometer inserted into the thickest part of the roast.
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Rinse off the seasonings that come on the corned beef. Corned beef has a distinct flavor and often no other seasonings than onion and vegetables are needed. Use a roaster oven for cooking a whole corned beef roast, which can weigh from 8 to 12 lbs.
Trim off most of the surface fat off the brisket with a sharp knife. Rinse the meat and pat it dry with paper towels. Allow the roast to sit for 30 minutes on the counter to come to room temperature.
Preheat the roaster for 10 to 15 minutes at 250 degrees Fahrenheit.
Place the roast in the roaster pan that fits into the electric roaster. Add the liquid, such as water, apple juice or cider, or a dark beer.
Put the roasting pan in the roaster, and put on the lid.
Test the temperature of the corned beef after 4 hours. Once it has reached about 140 degrees Fahrenheit, put the cabbage, potatoes, onions, carrots and rutabaga around the roast. Put the lid back on, and let the roast bake for another hour.
Remove the corned beef from the roaster, and put it on a platter. Cover it loosely with foil and let it sit for 15 minutes before slicing.
Drain the vegetables and serve them beside the sliced corned beef, or serve them as a soup with the broth from the roast.