Preheat the oven to 450 degrees F (230 degrees C). Line a roasting pan with aluminum foil.

Liberally coat roast with mustard so that it is entirely covered. Put a thick layer of rock salt on the foil in the roasting pan, about the size of the bottom of roast. Set roast on salt and press down to embed salt into mustard. Completely cover roast with rock salt, pressing it into mustard. (A lot of salt will fall off into the pan, but that doesn’t hurt anything).

Bake, uncovered, in the preheated oven until internal temperature reaches 145 degrees F (65 degrees C), about 1 hour. Let stand for 15 minutes.

Meanwhile, whip cream in a mixing bowl until stiff peaks form. Fold in horseradish to taste.

The roast will be encased in a shell of salt. Hit the shell sharply with the back of a knife; it will crack open and start to fall off. Remove the shell.

Place roast on a cutting board. Slice and serve with horseradish sauce.