From my cookbook Steak - now sadly out of print - a homemade alternative to demi-glace, a foolproof steak sauce that you can use on its own or as a basis for another dish such as bavette aux échalotes
Enough for 2-3 steaks
1 tbsp olive oil
15g (1/2 oz) butter
110g (4 oz) shallots or onion, peeled and roughly sliced
125ml (4 1/2 fl oz) red wine
1 tbsp balsamic vinegar
150ml (5 oz) beef stock, fresh or made with an organic beef stock cube
1 tsp butter paste*
Salt, pepper and Worcestershire sauce to taste
Heat the oil in a pan then add the butter. Once it has melted add the shallots, stir and cook for about 10 minutes until lightly browned, stirring occasionally. Add the red wine and balsamic vinegar, bring to the boil, turn the heat down and simmer for about 10 minutes, stirring occasionally until the liquid has reduced by about three-quarters. Add the stock and simmer for another 5 minutes. Strain, return to the pan and whisk in the butter paste with a wire whisk. Bring back to the boil and simmer until thickened. Season to taste with salt, pepper and a few drops of Worcestershire sauce if you think it needs it.
* to make butter paste mash together equal quantities of soft butter and plain flour until you have a smooth paste.