Heat the beef fat and oil in a heavy-based pan and brown the beef trim. Stir in the onions, star anise, peppercorns, carrots, mushrooms and garlic cloves.
Brown the meat and vegetables over a medium/high heat for 5 minutes of so.
Now use the half the port and half the red wine to deglaze the pan, scraping any crusty bits from the bottom of the pan.
Reduce the wine and the port for a few minutes.
Add the bones to the pan, followed by the stock and enough cold water to cover.
Bring to the boil, skim off any scum.
Simmer for two hours, strain through a chinois, pressing to extract as much liquid as possible.
Leave to cool and remove any solidified fat.
Bring to the boil, skimming again, and reduce by about three quarters until thickened.
Remove the reduced stock from the pan and pour in the remaining red wine and port and reduce again until syrupy and return the stock to the red wine reduction.
Add the fresh parley and tarragon followed by the cold butter, whisking it in to emulsify.
Allow the herbs to infuse briefly before straining again and serving.
