Starting about 48 hours in advance, line the bottom of a large roasting pan with onions and garlic. Lay the brisket on top.
Pour soy sauce over the brisket, sprinkle with onion soup mix and rub all over the surface of the meat.
Cover with plastic wrap and refrigerate overnight.
When you’re ready to cook, take the meat out of the fridge to take the chill off while you preheat the oven to 300°F.
Cover the pan tightly with a lid or foil and roast for 3 hours, or until very tender.
Set it aside to cool, then cover and refrigerate overnight. (Alternatively, let it rest for 20 minutes, then slice and serve right away with the onions and pan juices.)
To reheat the brisket for dinner, preheat oven to 325°F, remove any solidified fat from the brisket, slice the cold meat across the grain and spread slices out in the roasting pan (or another baking dish) with the onions, garlic and liquid.
Bake for 20 to 30 minutes or until heated through.