Sear the Beef
Begin by seasoning the beef generously with kosher salt and black pepper. Heat vegetable oil in a large skillet over medium-high heat. Brown each chunk of beef on one side, leaving it undisturbed for about 5 minutes until deeply browned. After searing, transfer the beef chunks to a 6- to 8-quart slow cooker. Repeat with the remaining pieces of beef. While it’s not necessary to brown all sides, this step adds rich flavor to the roast.
Prepare the Braising Liquid
In the same skillet, add the pearl onions and garlic. Season lightly with salt and generously with black pepper. Cook, stirring occasionally, until softened, about 3 minutes. Pour in the red wine, red wine vinegar, and ½ cup of water, scraping up any browned bits from the bottom of the pan. Bring the mixture to a boil, then pour it over the beef in the slow cooker. Add the herb sprigs, cover the cooker, and set it to cook on low for 4 hours.
Pickled Onions (Optional)
While the roast is cooking, prepare the pickled onions if desired. In a small saucepan, bring the red wine vinegar and sugar to a boil over high heat. Add the pearl onions along with a pinch of kosher salt. Stir until the sugar dissolves and the onions are heated through, about 1 minute. Remove from heat and allow the onions to pickle at room temperature for at least 1 hour. For best results, prepare the pickled onions in advance—up to 5 days ahead—and store them in a sealed container in the refrigerator.
Add Vegetables and Continue Cooking
After 4 hours, uncover the slow cooker and flip the beef chunks with tongs. Add the root vegetables, ensuring they are submerged in the liquid. Cover again and cook on low for an additional 4 to 5 hours, or until both the beef and vegetables are very tender. Test the beef with a fork to check for tenderness; it should easily pull apart. If the beef is not yet tender, continue cooking for another 1 to 2 hours. Once ready, remove the beef and vegetables from the slow cooker and transfer them to a serving platter. Break the beef into large chunks and drizzle with some of the cooking liquid. Top with the pickled onions.
Make Gravy (Optional)
For a thicker gravy, strain the cooking liquid into a medium pot and bring to a boil over medium-high heat. In a small bowl, combine the butter and flour, kneading them together until uniform. Drop the butter-flour mixture into the boiling liquid, whisking constantly to combine. Let it simmer for about 5 minutes, or until the gravy thickens. Serve the gravy over the pot roast and vegetables, and garnish with the pickled onions. Extra gravy can be served on the side for guests to help themselves.