Heat a large Dutch oven, with a lid, over medium-high heat. Season the beef generously with salt and pepper and sprinkle lightly with the flour. Add the oil to the pot, and sear the beef on both sides until a deep mahogany brown, about 10 minutes total.
Transfer the beef to a plate. Pour off all but about 2 tablespoons of the oil from the pot.
Add the garlic and onion to the pot and cook, stirring occasionally, until fragrant and soft, about 5 minutes. Add the tomatoes, breaking them up with your hands as you add them, and cook until a deep brick red, about 5 minutes more; reserve the tomato juice in the can.
Add the wine to the pot and use a wooden spoon to scrape up any browned bits that cling to the bottom. Add the reserved tomato juice, beef base, 2 cups water, thyme and bay leaves and bring to a boil. Return the beef to the pot, nestle it in the liquid, cover and transfer the pot to the oven.
Cook until the beef is just tender, about 1 1/2 hours. Remove the lid and continue to cook, uncovered until tender, about 1 hour more.
For the roasted vegetables: Meanwhile, toss the carrots, onions, parsnips and turnip with the olive oil and some salt and pepper in a large bowl. Spread out the vegetables in a single layer on a baking sheet. You may have to use 2 baking sheets. Once the beef comes out of the oven, raise the temperature to 450 degrees F. Roast the vegetables, turning halfway through cooking, until caramelized and tender, 30 to 45 minutes.
Transfer the beef to a plate and cover loosely with foil. Skim the fat off the surface of the sauce in the pot and discard. Bring the sauce to a boil over medium-high heat and cook until thickened, about 5 minutes. For a thicker sauce, whisk in the cornstarch mixture and bring to a boil. (Test the thickness of the sauce by spooning some onto a plate and checking the consistency). Stir in the parsley, if using. Taste and adjust the salt and pepper as needed. Keep the beef warm in the sauce until ready to slice.
Slice the pot roast, lay it on a platter and surround it with the vegetables. Spoon some of the sauce on top and serve the remaining sauce on the side.
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