Add the beef stock and bring the mixture to a gentle simmer. Add Thyme.
1 L Beef Stock
3 sprigs Thyme
Let it bubble away for about 20 minutes, uncovered, so the flavors can concentrate and the sauce begins to thicken slightly.
Return the Beef
Add the seared beef and any juices back into the pot.
Stir in a splash of apple cider vinegar—this helps tenderize the meat and adds a subtle brightness to balance the richness.
Simmer for another 25 minutes, uncovered, until the beef is tender and the sauce is luscious.
Finish with Sour Cream
Turn the heat to low and stir in the sour cream.
Let it gently simmer until the sauce is creamy and just the right consistency.
Taste and season with salt and freshly ground black pepper.