In a pan, preferably the pan you just made a steak or 2 in (leave the bits and residue from the steak), add the cranberry juice, mustard, Worcestershire sauce, shallots, a pinch of salt and a few grinds of pepper, then a few more grinds of pepper. Simmer on medium heat, stirring with a wooden spoon, until the liquid reduces slightly and thickens, about 5 minutes.
Strain the sauce with a fine-mesh sieve. While still warm, whisk in the butter until melted. Taste and add a squirt of agave nectar at a time to mellow the tart flavor of the cranberry juice. Serve warm.