Saute the shallots in a saucepan with a bit of butter
1 tablespoon butter, ½ cup shallots
When the shallots are soft, add port and red wine. Cook over medium heat until sauce has reduced by about half.
1 cup tawny port, 1 cup red wine
Add beef stock and rosemary
1 cup beef broth, 1 sprig rosemary
Continue simmering until sauce has reduced to about ⅓ -½ cup
Strain sauce discarding shallots and rosemary
Return sauce to pan and bring back to a simmer. Reduce heat to low and whisk in remaining butter one tablespoon at a time
2 tablespoons butter
Add a dash of salt and pepper and serve warm
salt, pepper
Notes
Yield is between ⅓ cup and ½ cup of finished sauce
Use a port and red wine that are enjoyable to drink. The flavor is the base for your sauce.
Once the sauce has fully reduced and you've strained it, you can cover and refrigerate for up to 2 days. Then pull it out when you're ready, heat it and whisk in the butter.
When adding the butter, reduce the heat to a very low heat or turn off the heat altogether. Add the butter slowly and whisk, whisk, whisk.
Taste sauce and add just a dash of salt and pepper if needed