Heat the oven to 300°F with a rack in the lower-middle position. Using kitchen twine, tie the roast at 2-inch intervals. Season on all sides with salt and pepper.

In a large Dutch oven over medium-high, heat the oil until shimmering. Add the onion and ¼ teaspoon salt, then cook, stirring, until the onion is well browned, 5 to 7 minutes. Stir in the garlic and cook until fragrant, about 30 seconds.

Add the tomato paste and cook, stirring constantly, until it begins to brown, about 2 minutes. Stir in the brown sugar, cinnamon, allspice, peppercorns and cloves. Pour in the wine, bring to a simmer and cook until thick and syrupy, 3 to 5 minutes. Stir in the Worcestershire sauce and prunes.

Place the roast in the pot, then turn to coat with the liquid. Cover and bake until a paring knife inserted into the thickest part meets no resistance, 3½ hours. Transfer the roast to a shallow baking dish and loosely tent with foil. Let rest for 30 minutes.

Meanwhile, set a fine mesh strainer over a medium bowl. Pour the contents of the pot into the strainer and press on the solids with a rubber spatula to extract as much liquid and pulp as possible; scrape the underside of the strainer to collect the pulp. Discard the solids.

Let the liquid and pulp settle for about 5 minutes (you should have about 1½ cups), then skim off the fat. Return the defatted liquid and pulp to the Dutch oven and bring to a simmer over medium.

In a small bowl, stir together 3 tablespoons water and the cornstarch. Whisk into the simmering liquid and cook, stirring constantly, until lightly thickened, about 2 minutes. Stir in the vinegar. Taste and season with salt and pepper.

Transfer the roast to a cutting board. Remove and discard the twine. Cut the meat against the grain into ½-inch slices and transfer to a platter. Pour about 1 cup of the sauce over the meat. Serve with the remaining sauce on the side.