Dust the beef cubes in seasoned flour. In a medium-sized casserole pot, heat the oil and brown the beef cubes on all sides, remove and set aside. Fry the onion, carrots and celery for 5 minutes. Add the chopped tomatoes, cherry tomatoes, beef stock pot, water and rosemary. Bring to a gentle simmer, add the beef cubes and sugar and simmer gently for 90 minutes. Add the cannellini beans and frozen peas and simmer for 10 minutes. Remove and season with salt and pepper.
While the stew is cooking, make the mash. Place the potatoes in a pot, and cover with cold water. Bring to the boil and boil for 30 minutes or until soft. Mash with a potato masher and whisk in the butter, milk and mustard until smooth. Season with salt and pepper.
Serve the stew on top of the mustard mash and sprinkle over chopped parsley and lemon zest.