Corned beef is best when cooking slowly and with some moisture, more or less braising. Limit the number of times you open your roaster because heat will quickly escape. In an electric roaster, cook the corned beef brisket at 350 degrees; a large roast (6-7 pounds) will take about 5 hours. The cook time for the brisket will depend on your roaster and how often you open the door/lid.

The corned beef is finished when it reaches an internal temperature, measured with a meat thermometer, of at least 145 degrees minimum but it will taste best when 160 degrees and fork-tender.

Preheat roaster oven for 15-20 minutes before adding the meat.

Braising Liquid
Join foil strips, using a butcher wrap to seal the joins, make a piece that will line the whole roaster and wrap completely around the roast and future vegetables.
Place your brisket, fat side up, on overlapping sheets of aluminum foil. Place in roaster pan, then add enough liquid to cover 3/4-1 inch of the way up the roast, about 2 cups. For your liquid, use water, apple cider, apple juice, water with cider vinegar, wine, etc.; it really depends on what you like. Add some orange peels (white pith removed), whole cloves, and 2-3 whole bay leaves, or the spice packet that comes with the roast. Wrap roast by folding top edges together (so you can peak inside to see the roast easily).