Brown the beef trim in the beef fat, add the onions and cook down with a pinch of salt
Add the carrots, leeks, thyme and garlic. Soften all the veg really well
In a separate pan reduce the port and wine by ½ then deglaze the beef/veg pan with this
Add the stock and allow to simmer/reduce slowly for 2 hours until a nice coating consistency forms
Strain and finish the sauce by adding seasoning if needed and some acidity like sherry vinegar or lemon juice
