Why It Works
- Separating the roast from the rib bones prior to cooking allows for more even seasoning of the meat and easier carving at the end. Tying the roast back onto the bones for cooking helps insulate the meat, keeping it moist while promoting even cooking.
- Slathering the roast with funky shio koji amplifies the beefy flavor of prime rib, imparting savory depth to the meat, while also tenderizing it. The sugars in the shio koji also help the prime rib achieve a well-browned crust.
- Slow-roasting in a low oven cooks the prime rib evenly from edge to edge.
Shio koji is a funky, salty, and slightly sweet fermented Japanese marinade made with koji, the delicious mold that also gives us miso and soy sauce. Rich in protease enzymes, shio koji is able to season and tenderize large cuts, unlike other marinades that only achieve surface-level flavor. For this recipe, we use it in the same way that we would deploy salt for a dry-brined roast—spreading shio koji over the surface of the prime rib and letting it rest uncovered in the fridge overnight before cooking.
The salt in the shio koji seasons the roast, which is then cooked in a low oven using the reverse-sear method before getting a final blast of intense heat to produce a crisp, well-browned crust, thanks to the sugars in the marinade. Served with shio koji-spiked beef jus, this is the next chapter in the Serious Eats prime rib canon.
The Day Before Roasting, for the Prime Rib: Using a sharp knife, cut meat away from bone in one piece, cutting as close to the bone as possible, leaving you with a boneless roast and a slab of rib bones; reserve rib bones. Pat roast dry with paper towels; then using your hands, slather shio koji over entire surface of the roast and rib bones. Place roast back on bones, and, using butcher's twine, tie together so that roast looks as it did before meat was removed from the bones. Transfer roast, fat cap side up, to a wire rack set in a foil-lined rimmed baking sheet, then refrigerate, uncovered, at least 12 hours and up to 24 hours.
When Ready to Cook: Adjust oven rack to center position and preheat oven to 225°F (105°C). Using a paper towel, gently wipe away excess bits of shio koji from the fat cap side of the roast. Transfer baking sheet with rack and roast to oven and cook until internal temperature registers 125°F (52°C) on an instant-read thermometer at the roast's thickest point for medium-rare or 135°F (57°C) for medium, 4 to 5 hours.
Remove roast from oven, tent loosely with foil, and let rest at room temperature for at least 30 minutes and up to 1 1/2 hours. Meanwhile, increase oven temperature to 500°F (260°C) or highest possible temperature.
Ten minutes before guests are ready to be served, remove foil, place roast back in hot oven, and cook until deeply browned and crisp on the exterior, 6 to 10 minutes. Remove from oven, and transfer roast to a carving board.
For Serving: Cut away twine, and separate roast from ribs. Using a sharp knife, cut between ribs to separate into individual pieces, and then slice roast into 3/4-inch-thick slices. Serve right away, passing coarse sea salt and koji beef jus (if using) at the table.
This recipe works for prime rib roasts of any size, from three to six ribs. For best results, use a dry-aged prime-grade or grass-fed roast. Cooking time is identical regardless of the number of ribs on the roast.
We do not recommend using store-bought prepared shio koji for any recipe, as store-bought versions vary wildly in quality. They are often too sweet and are treated with alcohol to extend shelf life. Making your own shio koji is worth the effort.
Make-Ahead and Storage
This recipe is best enjoyed right away.