Put a shallow pan over a low heat and melt half the butter. Add the shallots and sweat gently for a couple of minutes until softened.
Add half the red wine and turn the heat up high to bubble and boil away. Make a mental note of where the wine fills up the pan to, and then allow it to bubble furiously until reduced by half. Then add the rest of the wine and continue to reduce.
When the sauce has reduced again by half add the stock and stir to blend in for a minute. It's finished at this point - when ready to serve take off the heat. Add the remaining butter and swirl the pan until it has melted. If you want a smooth sauce strain off the shallot, otherwise serve as is. Check for seasoning before serving.
Notes
This sauce can be made a couple of days in advance and zapped in the microwave to serve, just hold back the finishing butter. It'll freeze good too.