Preheat the oven to 200°C conventional oven/ 180°C fan-forced (400° Fahrenheit/Gas 6). Line a large baking tray with baking paper.
To stuff the skirt (flank) steak, lay the pieces of skirt steak slightly overlapping on top of a large sheet of baking paper. Cover the meat with a sheet of plastic wrap and use a meat mallet or rolling pin to pound the meat to tenderise to an even thickness. Season both sides with sea salt and pepper. Arrange the Prosciutto slices evenly over the meat, leaving a 2cm border around the edges. Top the Prosciutto with mozzarella slices, then a layer of spinach leaves. Use the baking paper to roll the meat with the grain. Lay the meat seam-side down and use kitchen twine to tie the rolled skirt steak with individual pieces of string approximately 5 cm apart.
Heat the olive oil in a large oven-proof frying pan over medium heat. Add the garlic clove and the stuffed skirt (flank) steak roast. Sear the beef on all sides (approximately 6 minutes). Add the red wine to deglaze the pot and cook until slightly reduced.
Transfer browned meat and wine juices to the prepared baking tray. Add the red onion, carrots, celery, bay leaf and rosemary. Pour the hot beef stock over the roast. Cook for 30 minutes for medium cooked or until the beef is cooked to your liking. Transfer the roast to a chopping board to rest.
Pour the cooking liquid with vegetables to a small saucepan. Cook until it reaches a thick and rich gravy consistency (approximately 20 minutes). Strain the sauce through a sieve into a gravy boat, using the back of the ladle to extract as much flavor as possible. Discard vegetables.
Serve the Stuffed skirt (flank) steak roast with Prosciutto, mozzarella and spinach with Creamy mashed potato and gravy – buon appetito.