Preheat oven to 350°F (175°C). Using a sharp knife, slice the Hawaiian rolls horizontally, keeping the top attached if using as a full tray or separating tops and bottoms if making individual sandwiches. Place bottom halves on a baking sheet lined with parchment.

In a small bowl mix softened butter and Dijon mustard until emulsified. Brush butter-mustard mixture evenly over the cut side of each roll (both top and bottom if separated). Sprinkle the tops lightly with kosher salt and freshly ground black pepper.

If your prime rib is cold, warm it gently: place the thin slices in a single layer on a baking tray, cover loosely with foil and warm in the oven for 6–8 minutes until just heated through (do not overcook). Alternatively, warm slices in a skillet over medium-low heat with a teaspoon of beef drippings for 2–3 minutes, turning once.

On each bottom roll layer approximately 1.6 oz (about 1–2 slices) of warmed prime rib, folding slices as needed to fit evenly across 12 rolls.

Tear provolone slices in halves or quarters and place over the prime rib so each sandwich gets about 0.5 slice (total 6 slices covers 12 small rolls).

Place the top halves of the rolls over the sandwiches. Brush tops lightly with any remaining butter-mustard mixture.

Cover the assembled tray loosely with foil and bake at 350°F (175°C) for 8–10 minutes, until cheese is melted and rolls are warmed through. Remove foil for the last 1–2 minutes if you want slightly crisped tops.

Finishing & Serving

Remove from oven and transfer sandwiches to a cutting board if assembled as a tray. Using a serrated knife cut between rolls into individual sandwiches if needed.

Serve immediately with a generous dollop of the creamy horseradish sauce on the side or spread inside before adding the top roll. Garnish with chopped parsley if using.

If serving au jus: reheat beef au jus or beef broth with drippings and Worcestershire until simmering in a small saucepan. Place a small ramekin of au jus beside each plate for dipping.