• Using a sharp knife, cut slits, spaced 1 inch apart, in the surface layer of fat cap in crosshatch pattern, being careful to cut down to, but not into, the meat. Rub 2 tablespoons kosher salt over entire roast and into the slits. Transfer to a large plate, and refrigerate, uncovered, for at least 24 hours or up to 4 days.
• Take the roast out of the refrigerator and let it sit on the counter for an hour to raise the roast temperature. To help cook the roast evenly, tie the roast with kitchen twine parallel to the rib bones at least at each end or tie between each pair of ribs.
• Adjust oven rack to middle position and heat the oven to 200°F. Set a wire rack in a rimmed baking sheet. Heat oil in a 12-inch skillet until just smoking. Sear sides and top of the roast until browned, 6 to 8 minutes total. Let meat cool for 10 minutes. Transfer roast, fat side up, to a roast pan with a wire rack and season with pepper, garlic and rosemary. Insert thermometer-probe in the center of the roast without touching any bone. I roasted it until the meat registered 120°.
• Remove roast from oven but leave it on the baking sheet, do not remove the thermometer-probe, tent loosely with aluminum foil, and let rest for another 20-30 minutes. The heat carry-over will bring the meat to 135 for medium-rare and will be tender and juicy.

For the Sauce: (Beurre Manié)

• Add the beef broth, chicken broth and red wine to a saucepan and over medium-high heat bring it to a boil, decrease the heat to low and simmer until the mixture is reduced to 2 cups.
• In a small dish add the butter and flour and mash them together with the back of a tablespoon to make a paste. Remove the reduced broth mixture from the heat and add the paste, whisking together to melt the paste. Set the saucepan back on the heat and boil briefly, whisking continuously for another minute until thickened. Transfer to a serving pitcher and pass to guests.