Place all ingredients except white sauce, mustard and parsley in your chosen cooker and cover with water. Slow cook on low for 6-8 hours, pressure cook for 45 minutes or stove top simmer on lowest temperature in a large covered pot for 2 hours. If using stovetop, you have to make sure the temperature is low enough to get a light simmer or your corned beef will come out tough.
Make white sauce according to recipe and add in mustard and parsley. Mix to combine. Remove corned beef and slice. Serve topped with mustard parsley white sauce. Green beans are a great side with this dish and making your veg at the end of cooking in the same pot as your corned beef is a good option too.White Sauce recipe here