Put meat in Dutch oven with onions. Add the thyme and sprinkle with mustard powder. Add water, pouring down the side of the pan, not over the corned beef, to a level of about 3/4 up the sides of the corned beef, but not over, and bring just to a boil. Immediately turn down to a simmer and skim off any foam.
Add a lid and simmer very gently (small bubbles popping up here and there on a regular basis) for an hour and a half. Add the vegetables, cabbage on top and simmer gently another hour. The liquid level may need to be checked during the long simmer.
When finished, the Corned beef should be tender but not stringy and falling apart, the internal temperature should be 160 degrees. Rest for 15 to 20 minutes before cutting into thin slices across the grain. Serve on a platter with the vegetables, pour a little of the broth over and pass the remaining. Garnish with parsley if desired.
Note: If you’d like a lot of vegetables, cook the corned beef until done, remove to rest, then add as many vegetables as you can fit in the pot, potatoes on the bottom, cabbage on top. Cover and simmer briskly for 20 – to 25 minutes until vegetables are tender.
Corned Beef in the Slowcooker:
If you need to cook this in a crockpot, throw everything together, rinsed Corned Beef on the bottom along with the onions, add the thyme, sprinkle with mustard powder, then the contents of the spice packet. Then either:
Serve on a platter with the vegetables, pour a little of the broth over and pass the remaining. Garnish with parsley if desired.
If serving 4, there should be leftovers.
The general rule of thumb for braising different weights of corned beef: Cook 30 minutes, then add an additional 30 minutes per pound. Smaller cuts may need slightly less time.