Preheat the oven to 300˚F.
Set an ovenproof pot or Dutch oven over medium-high heat, add a drizzle of oil and brown the roast on all sides.
Add the onions to the pan and cook for a few minutes, until starting to brown.
Add the tomatoes, Worcestershire sauce and enough water to almost cover the meat, cover and cook for 3-5 hours, or until the beef is very tender. (Alternatively, cook in a slow cooker on low for 4-8 hours, then transfer the meat to a baking dish.)
Pull the meat apart with forks, stir in the carrot and peas and top with the mashed potatoes, smoothing the top.
Turn the oven up to 375˚F and bake for another 15-20 minutes, until bubbling and golden.