It's remarkable that a pork roast can be so good so quickly! The gravy is rich and flavorful and the meat is moist and perfectly cooked--using an Instant Pot is a real game changer for pork loin roast. Source: 400 Calorie Recipes
Season pork on all sides with 1 tablespoon rosemary, 3/4 teaspoon salt, and 1/2 teaspoon pepper. Add oil to a programmable pressure multicooker (such as Instant Pot). Select SAUTE setting. Select HIGH temperature setting, and allow to preheat. Add pork; cook on all sides until nicely browned, about 3 minutes per side. Transfer pork to a plate. Pour off excess drippings in cooker; do not wipe cooker clean. Add onion, garlic, 1/2 teaspoon of the rosemary, and remaining 1/2 teaspoon each salt and pepper to cooker; cook, stirring constantly, until onion softens, about 1 minute. Add stock, stirring to scrape up any browned bits from bottom of cooker.
Return pork to cooker. Press CANCEL. Cover cooker with lid, and lock in place. Turn steam release handle to SEALING position. Select MANUAL/PRESSURE COOK setting. Select HIGH pressure for 17 minutes. (It will take 6 to 7 minutes for cooker to come up to pressure before cooking begins.)
Carefully turn steam handle to VENTING position, and let steam fully escape. (This will take 3 to 4 minutes.) Remove lid from cooker. Transfer pork to a cutting board; let rest 10 to 12 minutes.
While pork rests, whisk together cornstarch and cold water in a small bowl until combined. Select SAUTE setting on cooker. Select HIGH temperature setting, and let liquid mixture in cooker come to a boil. Add cornstarch mixture, whisking until combined. Cook, whisking constantly, until mixture thickens into gravy, 1 to 2 minutes. Add butter and remaining 2 1/2 teaspoons rosemary to gravy, stirring until butter is melted. Turn cooker off.
Slice pork into 3/4-inch pieces. Arrange slices evenly among 6 plates; spoon gravy over pork.