Heat the sunflower oil in a large casserole dish set over a moderate heat. Season then seal the beef until golden in colour all over. Remove from the dish to a colander set over a bowl.
Add the pancetta to the dish and cook for a few minutes, stirring occasionally. Add the red wine and deglaze the base of the dish. Allow it to reduce by half.
Add the beef back and sprinkle the flour on top. Stir a couple of times, then add the pearl onions, stock and bay leaf. Bring to the boil, then cover and reduce to a simmer. Simmer for 80 minutes, then remove the lid and remove the beef from the sauce. Place in a bowl and cover with aluminium foil to keep warm. Reduce the sauce until it had thickened. Add the beef back to the dish to warm through before serving