Heat the butter and oil in a large saucepan over a medium-high heat and fry the shallots, carrot and celery for 7-10 mins, or until deep golden brown. Tip in the flour and cook, stirring, for 1 min. Pour in the wine and stir to combine everything, then simmer on a low heat for a few minutes, or until the wine has reduced by half. Add the rosemary and bay leaf, then gradually stir in the lamb stock. Simmer for 30 mins, uncovered.

Strain the gravy through a fine sieve into a clean saucepan. Stir in the Worcestershire sauce and simmer on a low heat until thick and glossy, about 15-20 mins. Will keep frozen for three months.