Place the corned beef into a baking dish with a lid. In a bowl, whisk together the remaining brisket ingredients. Pour the liquid over the brisket and roll the meat to coat with the sauce. Cover and bake for 1 hour.
Flip the meat over, cover, and bake another hour.
Uncover the meat and make sure the fat side is on top. Bake uncovered for the last hour, until it is tender enough to put a fork into it easily. Baste with the sauce every 15 minutes as it cooks to form a glaze or “bark” on the meat.
Remove from heat and allow to cool fully before slicing, ideally overnight in the fridge. Make sure you keep the excess sauce!
Put the onion in a jar and add the remaining ingredients. Add a little water to top it off. Stir well and cover. Allow to rest on the counter for a few hours, then in the fridge until ready to use.
Make the mac and cheese according to the package instructions - boil the noodles, strain, then mix with the butter, milk, and seasoning packet.
Slice the brisket thin. Heat a frying pan to medium heat. Add some oil and add in the brisket to sear on both sides. Meanwhile warm up the extra sauce in the microwave.
Build the sandwiches, starting with the bottom bun, then some sauce, then the brisket, then the mac and cheese, a few onion slices, then finally the top bun. Serve.