In a medium saucepan, combine the beef stock, red wine, and tomato paste.

Stir in the dried thyme and black peppercorns.

Place the saucepan over medium heat and bring the mixture to a gentle simmer.

Let it simmer for about 15-20 minutes, stirring occasionally.

Once reduced, remove the saucepan from heat and strain the sauce through a fine mesh sieve into a clean bowl.

Whisk in the unsalted butter until melted and fully incorporated.

Taste the sauce and season with salt as needed.

Use the demi glace immediately or allow it to cool before storing in an airtight container.