Drain and discard extra liquid from corned beef package, keeping the seasoning packet if there is one.* Rinse the corned beef to remove any pink goop and pat dry with paper towels.
Rub corned beef evenly with spice packet contents (if there is one*).
Heat oil in a dutch oven over medium-high heat.
Place corned beef in dutch oven and sear on both sides for 2-3 minutes.
Flip the meat over so the fatty side of the meat is facing up and add broth, beer, Worcestershire sauce and bay leaves to the dutch oven and stir to mix in around the corned beef. Add additional water if needed until the top of the corned beef is just barely covered in liquid. It may float to the surface which is fine, no need to add more liquid.
Bring to a low simmer (do not boil!), cover and let it cook for about 40-45 minutes per lb. until meat feels tender (exact time will depend on exact cooking temperature and size for brisket). Lift the lid to check several times during cook time to make sure the pot is still at a low simmer and adjust the burner accordingly.
While the corned beef cooks, prepare the vegetables. Cut potatoes in half, quarters or smaller until they are about 2 inches in size. Peel the carrots and cut each into 3-4 pieces. If any are really large, cut thick parts in half. They should be half the size of the potatoes. Slice onion into thin wedges and separate into individual slices. Slice cabbage into 2 inch thick wedges, removing and discarding the hard inner core at the bottom.
Once corned beef is fork tender, but not falling apart, add the vegetables. Place carrots, potatoes, and sliced onions alongside the brisket submerged in the liquid. Place cabbage on top, spooning liquid over the top of the cabbage if it isn’t submerged.
Cover and cook at a gentle simmer for 25-35 minutes until potatoes and carrots are cooked through and meat is fully cooked. You may need to increase the temperature to get the pot back to a low simmer after adding the vegetables.
Remove corned beef from the pot and place on a plate. Let the meat rest for 15 minutes.
While meat rests, add salt and pepper to season the vegetables and cooking liquid to taste. Then drain the vegetables reserving some of the cooking liquid (optional) to serve with the meal.
Use a very sharp knife to thinly slice corned beef against the grain (crosswise at 90° to how the lines in the meat naturally run).
Place vegetables around corned beef on a serving platter and serve with Dijon, stone ground mustard and/ or cream style horseradish and reserved pan liquid.