Preheat the oven to 200°C/gas mark 6. Make a series of small slits in the beef, and stuff each with a slice of garlic.
Add 1 tbsp oil to a large frying pan and seal the beef on all sides over a high heat. Place in a roasting tray and scatter the onions around it. Drizzle the onions with the remaining oil and season to taste. Roast for 20 minutes, then reduce the heat to 180°C/gas mark 4. Continue to cook for 20 minutes per 500g, removing the onions after the first 20 minutes to prevent them from burning.
Once the beef is cooked, wrap it in greaseproof paper and set it aside to rest for 15-20 minutes. Heat the roasting tray it was cooked in over a medium heat and add the flour. Cook for 2 minutes, then gradually add the stock, stirring constantly to prevent lumps. Once all the stock has been added, add the balsamic and simmer for 5 minutes, until thickened. Serve with the beef and onions.