Po Boy with sloppy roast beef

Preheat the oven to 160C/gas 3. Trim any hard fat from the beef and season on both sides with salt and lots of freshly ground black pepper. Heat 1 tablespoon of the oil in a large non-stick frying pan. Fry the beef over a medium-high heat for 8-10 minutes or until nicely browned on all sides, turning every 3-4 minutes. Transfer to a large flameproof casserole.

Return the frying pan to the hob and reduce the heat slightly. Add the remaining oil and fry the onion, celery and carrots for 5 minutes or until lightly browned, stirring regularly. Stir in the garlic for the last minute of cooking time. Tip into the casserole with the beef.

Pour half the stock into the frying pan and allow to bubble for a few seconds, stirring constantly, then pour over the meat and vegetables. Add the remaining stock and the tomato purée. Strip the thyme leaves from the stalks and scatter into the pan (or sprinkle over the dried thyme), add the bay leaf and stir well.

Bring the liquid to the boil and skim off any foam that floats to the surface. Cover the casserole and transfer carefully to the oven. Cook for 3½-4 hours or until the beef is very tender and falling apart. Turn the beef in the stock halfway through the cooking time to help ensure it cooks evenly.

Transfer the meat to a board and strain the cooking liquor through a fine sieve into a saucepan. Tilting the pan slightly, spoon off as much excess fat as possible - it will have floated to the surface.

Mix the cornflour and mustard powder with 2 tbsp cold water in a small bowl until smooth. Stir the cornflour mixture into the warm liquor and bring to a simmer. Cook for 3-5 minutes or until the sauce is well reduced and looks thick and glossy, stirring regularly.

Split each baguette horizontally almost half the way through and spread with mayonnaise. Carve the hot beef into rough slices and arrange on the bread. Spoon over a little of the gravy. Cover with sliced tomatoes and top with shredded lettuce. Serve immediately.