How to Make It
Make the pot roast In a large enameled cast-iron casserole, heat the olive oil. Season the meat with salt and pepper and dredge in flour. Working in 2 batches, sear the meat over moderately high heat until browned all over, 5 minutes per batch; transfer to a plate. Add the vegetables to the casserole and cook over moderate heat for 5 minutes. Return the meat and juices to the pot, add the wine and bring to a boil. Cover and cook over low heat, stirring occasionally, until the meat is tender, 1 hour and 30 minutes. Stir in the butter and vinegar; season with salt and pepper.
Make the pickled red onion Pack the sliced red onion into a 1-quart jar. In a saucepan, combine the vinegar, bay leaf, sugar, salt, peppercorns and 1 1/2 cups of water and bring to a boil until the sugar is dissolved. Pour the brine over the red onion and let cool; drain well.
Make the mashed potatoes In a medium saucepan, cover the potatoes and onion with cold water and bring to a boil. Add a big pinch of salt and simmer until the potatoes are very tender, 40 minutes. Drain the potatoes; return to the saucepan. Add the butter, season with salt and pepper and whisk until a chunky puree forms.
Spoon the mashed potatoes into shallow bowls. Top with the pot roast, garnish with the pickled red onion and parsley and drizzle with olive oil; serve.