Rinse the corned beef under cold water to get most of the salt brine off.
Cut onion, potatoes, and carrots into large pieces. Set potatoes and carrots aside.
Layer a 9"x13" glass dish with onions. Trim the fat off the brisket and place on top of onions.
Add one cup of beef broth to the glass dish.
In a bowl, mix the marinade together by combining brown sugar, dijon mustard, soy sauce, minced garlic, horseradish, and molasses.
Pour marinade over the over the onions and beef in the glass dish and cover with foil. Place in a 325 degree oven for 3 hours.
After 2 hours, add potatoes and carrots for the remaining hour. Cook until potatoes are cooked all the way through.
After 2 1/2 hours, steam the cabbage in a pot of water.
Serve hot when ready.