Cut ½ large sweet onion into large pieces and place at bottom of slow cooker along with water and ½ of the 1 cup brown sugar.
Remove 3 lb corned beef brisket from package and rinse briefly under cold water. I like to rinse some of the brine off because I find it too salty for my tastes. You don't have to do this part.
Place 3 lb corned beef brisket in slow cooker with the fatty side down. Don't trim the fat, it gives more flavor to the sauce.
In a bowl, mix remaining 1 cup brown sugar, 2 Tbsp stone ground mustard, 1 tsp salt & ½ tsp black pepper together. Smear mixture over entire corned beef.
Wash and quarter the 6 medium red potatoes and place on the sides of the slow cooker bowl around the seasoned corned beef.
Cut 1 whole green cabbage into large pieces and put on top of beef and potatoes in slow cooker.
For the slow cooker, set to low and let cook for 8 hours or until the beef becomes soft.
For pressure cooker, add 1 more cup of water and set to high for 30 minutes.