Heat the butter and vegetable oil in a casserole dish
Season the shanks to taste and brown well
De-glaze the pan with the port, stirring to ensure that all the little bits from the bottom of the pan have been lifted
Add the stock and bay leaves, cover and simmer gently for approximately 3 hours
Remove the shanks from the sauce and brush with a little of the olive oil
Roast the meat in the oven for approximately 1 hour, while simmering the sauce to reduce
Strain the sauce to a clean saucepan and correct the seasoning (add the mint jelly if using)
You may need to add a bit more port or sherry at this stage - if the sauce is too thin, add the cornflour mix and stir in to simmer for a minute or two, until thickened to your liking