In a small bowl, mix together the garlic, brown sugar, salt and pepper to taste. Pat the mixture all over the brisket then wrap the brisket tightly in plastic wrap and refrigerate for at least 8 hours or up to 2 days.
Unwrap the brisket and blot it dry with paper towels. Let it come to room temperature while the oven preheats to 325°F.
In a heavy-bottomed Dutch oven, heat the oil over medium-high heat for 2 minutes. Add the brisket and brown on all sides, about 8 minutes total. Transfer the brisket to a plate.
Reduce the heat to medium and add the onions. Cook, stirring often, just until the onions start to soften, 3-4 minutes. Add the bay leaves, 1/4 cup of apple cider and the orange juice. Stir and scrape up any browned bits from the bottom of the pan. Add the remaining 1 1/4 cups cider and the thyme sprigs to the pot, increase the heat to high and bring to a simmer. Add the cabbage and the apples then nestle the brisket in the middle, fatty side up. Cover the pot with a sheet of foil and place the lid on top to create an extra-tight seal.
Transfer pot to the oven and cook the brisket for 1 1/2 hours. Scoop some of the onions and cabbage up from the bottom of the pot over the top of the brisket then re-cover the pot with the foil and place the lid back on. Continue to braise the brisket another 2-2 1/2 hours, until a paring knife easily slips into the center of the meat without any resistance. Remove pot from the oven and set the brisket on a cutting board to rest for 10 minutes. Slice it against the grain into thin pieces.
Stir the mustard into the sauce in the Dutch oven and adjust the salt and pepper if needed. Serve the brisket slices covered with the sauce. Crusty bread on the side is always a nice addition to help scoop up lost bits of meat and juicy apples and onions.
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