1) Preheat the oven to 180C/Gas 4.
2) Heat a large casserole pot, with a lid, over medium-high heat. Season the meat generously with salt and pepper, and sprinkle lightly with the flour. Add the oil to the pot, lay the meat in the pan and sear on both sides until a deep mahogany brown, about 10 minutes. Transfer the meat to a plate. Pour all but about 2 tablespoons of the oil from the pan.
3) Add the onion and garlic to the pan, and cook until fragrant and soft, about 5 minutes. Add the tomatoes, breaking them up by hand as you add them to the pot, and cook until a deep brick red, about 2 minutes more. Add the wine, and with a wooden spoon scrape up any browned bits that cling to the bottom of the pot. Add the remaining tomato liquid, beef broth, thyme, and bay leaves, and bring to a boil. Return the roast to the pot, nestle it in the liquid, cover and transfer pot to the oven.
4) Cook until the roast is just tender, about 1 1/2 hours. Remove the lid and continue to cook, uncovered until tender about 1 hour more.
5) Meanwhile, toss the onions, carrots, parsnips, and turnips with the olive oil, salt, and pepper. Spread out on a baking sheet in 1 layer. You may have to use 2 baking tins. Once the roast comes out of the oven, raise the temperature to 230C/Gas 8. Roast the vegetables, turning about halfway through cooking, until caramelized and tender, about 30 to 45 minutes.
6) Transfer the roast to a plate and cover loosely with foil. Skim the fat off the surface of the liquid and discard. Bring the sauce to a boil over medium-high heat, and cook until thickened. For a thicker sauce whisk in the cornflour mixture and bring to a boil. (Test the thickness of the sauce by spooning some onto a plate and checking the consistency).
7) Stir in the parsley, taste and adjust salt and pepper, as needed. Keep the roast warm in the sauce until ready to slice.
8) Slice the pot roast and lay on a platter, surround with the vegetables. Pour some of the sauce on top and serve the remaining in a sauceboat on the side.