This delicious, affordable brisket proves that you don't have to blow your budget to have a special holiday feast.
Preheat the oven to 300°.
In a small bowl, mix 2 tbsp. kosher salt, 2 tbsp. black pepper, 6 of the chopped garlic cloves, and the thyme. Rub the mixture evenly on both sides of the brisket, pressing to adhere. In the bottom of a large roasting pan, arrange the onion slices in a single layer. Place the brisket on top, fattier-side up. Roast, uncovered, for 1 hour.
Meanwhile, in a medium bowl, whisk the stock and the Worcestershire. After an hour of cooking, remove the roasting pan from the oven and add the stock mixture. (You want about 1/3 inch of liquid in the pan; add water if you don’t have enough.) Cover the pan tightly with a double layer of foil, making sure there aren’t any gaps. Roast until the meat is fork-tender, about 5 hours more. Transfer the brisket to a platter. Tent with foil and let the meat rest while you make the mushrooms. Skim the fat from the surface of the liquid in the pan.
Increase the oven temperature to 450°. On a large rimmed baking sheet lined with parchment, toss the mushrooms with 3 tbsp. oil; season with salt and pepper. Spread the mushrooms in an even layer. Roast until tender and crispy at the edges, about 25 minutes.
In a small bowl, whisk the remaining chopped garlic, the parsley, rosemary, panko, the remaining 1 tbsp. oil, and the lemon zest and juice. Season with salt and pepper. Spoon the mushrooms with juices into a large bowl. Toss with the herb mixture.
Slice the brisket across the grain. Spoon some of the liquid from the pan over the meat. Serve with the mushrooms.