1. Pat the lamb dry with paper towels and season with salt and pepper.
2. Set the Instant Pot to sauté and add the oil. Once the oil is hot, brown the lamb all over. Remove the lamb and spread the top and sides with the garlic and rosemary.
3. Insert the rack into the bottom of the Instant Pot. Pour in the water into the bottom of the pot. Place the lamb on the rack and set the Instant Pot to meat/stew. Cook 30 to 35 minutes, depending on how rare you want it (30 minutes is good for medium rare on a 4lb leg of lamb). Let the pressure release naturally before removing the lid.
4. Preheat the broiler. Set the lamb on a broiling pan and broil 6 inches from the heat for about 2 minutes, to brown the top. Remove and let rest 10 minutes before slicing.