You can ask the butcher to butterfly the flank steak for you.
Method
Pre-heat the oven to 150°C/300°F.
Lightly coat the sirloin in olive oil and generously season with salt and freshly ground black pepper.
Heat a splash of olive oil in an oven-safe roasting pot over a high heat and brown all sides of the roast, including the ends, to seal the meat and lock in the juices. Remove and set aside.
Lower the heat (add more olive oil if necessary), add the onions, celery and carrots to the pot and gently fry until lightly caramelised.
Stir in the garlic, thyme, rosemary and bay leaves and cook for a further minute before mixing in the cranberries, balsamic vinegar and honey.
Make space for the roast by moving the vegetables to the sides and place the beef in the centre of the pot with the fatty side facing upward.
Carefully pour the beef stock around the roast, cover with a tight-fitting lid and cook in the oven for 2 hours and 45 minutes.
Stir in the mushroom quarters and cook for a further 15 minutes.
Remove the roast from the oven and transfer to a chopping board. Loosely cover with tinfoil and let it rest for a few minutes.
Carve the sirloin roast into slices, return to the pot and spoon meat, gravy and vegetables over steaming hot mashed potatoes.