Open corned beef package and rinse the raw brisket in cold water to wash off a majority of the salty brine.
Cut up whole onion into chunks and place in the bottom of the pressure cooker or crockpot.
Add corned beef on top of onions.
In a separate bowl, mix the marinade by combining brown sugar, dijon mustard, soy sauce, minced garlic, horseradish sauce, and molasses.
Pour marinade over corned beef and onions.
Add quartered potatoes on top of meat.
Next, layer the carrots on top then beef broth.
Seal lid on pressure cooker and cook for 40 minutes.
If you're using a slow cooker, then cook on high for 4 hours or low for 6 hours.
Just before the corned beef is done, steam the cabbage in a pot on the stovetop. Just with boiling water, salt, and pepper.
When everything is done, plate everything on a platter and serve with extra mustard or horseradish.
For adults, serve with your favorite dark beer.