Step 1

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.

Step 2

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.)

Step 3

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.

Step 4

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.

Step 5

Slice pork into 3/4-inch pieces. Arrange slices evenly among 6 plates; spoon gravy over pork.