Prepare the Lamb: Pat your boneless lamb roast completely dry with paper towels. Season it generously all over with salt and freshly ground black pepper.
Brown for Flavor: Set your Instant Pot to the 'Sauté' function and add the olive oil. Once the oil is hot, carefully place the lamb roast in the pot and brown it on all sides until golden, about 2-3 minutes per side. This step is key for a rich flavor!
Pressure Cook to Perfection: Place the metal trivet (rack) at the bottom of your Instant Pot and pour in the water or broth. Carefully transfer the browned lamb roast onto the trivet. Secure the lid, set the valve to 'Sealing,' and select the 'Meat/Stew' function (or Manual/Pressure Cook, High Pressure). Cook for 25-35 minutes, depending on your preferred doneness (30 minutes is ideal for a medium-rare 4lb roast).
Release and Check: Once cooking is complete, allow the pressure to release naturally for 10 minutes, then manually vent any remaining pressure. Carefully open the lid. Check the lamb's internal temperature with an instant-read thermometer; for medium-rare, aim for 135-140°F (57-60°C). If needed, you can return it to the pot for a few more minutes.
Crispy Finish (Optional): For a beautifully browned and slightly crispy top, preheat your broiler. Gently remove any netting from the lamb, then rub the minced garlic and chopped rosemary all over the roast. Transfer the lamb to a baking dish and broil for about 2 minutes, 6 inches from the heat, until the top is golden and aromatic.
Rest and Serve: Let the succulent lamb roast rest for about 10 minutes before slicing. This allows the juices to redistribute, ensuring maximum tenderness and flavor. Enjoy your perfectly cooked, healthy lamb!