Puree 1/4 cup orange juice concentrate, the tomato paste, mustard, chili sauce, sugar, ginger, garlic and five-spice powder in a food processor until smooth. Season the brisket with salt and pepper and rub with the spice paste. Place in a resealable plastic bag and refrigerate overnight.
Preheat the oven to 425 degrees F. Heat the vegetable oil in a large ovenproof pot over medium heat. Sear the brisket until golden, about 7 minutes per side, then transfer to a plate. Cook the onion in the pot until translucent, about 5 minutes. Add the leeks, carrots and parsnips, season with salt and pepper and cook until browned, about 8 minutes. Stir in the remaining 1 1/4 cups orange juice concentrate and the broth, scraping up the browned bits in the pot. Add the brisket and bring to a simmer.
Cover the pot, transfer to the oven and cook 1 hour, then reduce the heat to 325 degrees F and continue to cook until the meat is fork-tender, about 2 more hours. Transfer the brisket to a cutting board. Skim the fat from the sauce, then return the pot to the oven until the sauce thickens slightly, about 30 minutes. Slice the meat and serve with the sauce and vegetables. Garnish with scallions.