Add the butter and oil to a frying pan and heat over a medium heat.
When the butter has melted, add the shallots, salt, pepper, and garlic. Cook for 2-3 minutes until the shallots are just starting to soften.
Add the cognac to the pan, bubble for 1 minute, then add in the stock, Worcestershire sauce, and mustard. Cook for a further 2-3 minutes until slightly thickened.
Stir in the cream (if you have any juices from resting your steak, add them to the pan now too). Bring back to a simmer, then turn off the heat.
Serve over steak.