Decide which goal you need: closest flavor (reduce stock + add body) or fastest fix (bouillon + thickener).
If you have time, reduce beef stock until it coats the back of a spoon, then enrich with gelatin for a demi-glace-like cling.
If you need speed, simmer bouillon with a splash of Worcestershire and (optionally) a little wine, then thicken with a cornstarch slurry.
Warm the sauce gently and adjust thickness: reduce for body or add a teaspoon of water/stock to loosen.
Taste and balance with a tiny pinch of salt (if needed) and/or a few drops of Worcestershire to land in that savory, browned depth.
