Place the brisket, 2 tablespoons of kosher salt, bay leaves, 4 tablespoons brown sugar, garlic, and onions into a large pot. Add enough water to completely cover the beef. Turn the heat on high and bring the mixture to a boil. Skim any scum that rises to the surface. Reduce the heat to medium-low or a gentle simmer. Cook for 3 hours or until the beef is tender and cooked through.

Make the sauce: Add the oil, mustard, remaining brown sugar, ketchup, apple cider vinegar, and remaining salt and black pepper into a bowl. Mix well until combined into a smooth sauce. Set aside.

Preheat the oven to 350 degrees.

Gently transfer and drain the beef onto a paper towel lined cutting board and let it cool for about 5 minutes or until cool enough to handle. The liquid that is left behind in the pot is a beef broth that you can save for another dish. Pat the beef dry and slice the beef against the grain into ½ - inch thick slices. Use a large spatula or tongs to transfer the sliced corned beef onto a baking tray, while keeping its shape intact. Slightly fan out the beef slices on the baking tray and pour the sauce over the beef. Use a spatula or brush to evenly cover all the beef with sauce.

Cover the baking tray with aluminum foil and transfer the beef into the oven. Roast it for 30 minutes or until the beef is lightly browned and absorbs the sauce.

Serve hot.