Add the minced shallots and cook for 3 to 4 minutes until softened but not browned.Browning can make the finished sauce slightly bitter.
Deglaze with Port Wine
Pour in the port wine and stir to loosen any flavorful bits from the pan. Bring to a gentle simmer.
Keep the heat moderate. You want slow concentration, not harsh boiling.
Reduce the Wine
Simmer until the port reduces by about half and smells rich instead of alcoholic. This usually takes 8 to 10 minutes.The sauce flavor deepens dramatically during this step.
Add Demi-Glace & Aromatics
Stir in the demi-glace, and thyme. Simmer gently until the sauce coats the back of a spoon.
Taste & Balance
Taste the sauce. Add salt, pepper, or a splash of vinegar if it needs brightness. Great sauces are balanced, not just rich.
Finish with Butter
Reduce the heat to low and whisk in the cold butter, one cube at a time, until glossy and smooth.
Do not boil after adding butter, or the sauce may separate.
Strain & Serve
For an elegant restaurant-style finish, strain the sauce before serving over steak, pork, duck, or roasted meats.