How to Make It
Preheat oven to 375°F. Trim excess meat from bottoms of exposed rib bones on pork roast, and reserve for another use. Score fat on pork in a crosshatch pattern. Rub all sides of pork evenly with salt.
Place pork, fatty side down, in a cast-iron skillet; cook over medium-low, pushing pork down using a spatula to ensure even cooking, until fat renders and bottom side is browned, 15 to 20 minutes. Flip pork bone side down. Add butter to skillet; cook until foamy, 30 seconds to 1 minute. Stir in garlic and rosemary sprig. Baste melted butter over pork.
Transfer skillet to preheated oven. Roast until a thermometer inserted in thickest portion of meat registers 130°F, 45 to 55 minutes, removing skillet to baste pork with butter mixture every 10 minutes. Remove skillet from oven. Transfer pork to a wire rack set over a baking sheet; let rest 15 minutes.
Return butter mixture in skillet to stovetop; heat over medium-low. Add 1/3 cup water to skillet; cook, stirring constantly and scraping bottom of skillet to loosen browned bits, until liquid is slightly reduced, about 30 seconds. Add lemon juice; cook, stirring constantly, until incorporated, about 30 seconds. Remove and discard rosemary sprig.
Slice pork between bones, and arrange on a platter. Pour sauce over pork.