In a small mixing bowl, combine anchovies, butter, and salt. Beat together until anchovies are broken up and a smooth butter has formed. Set aside.
Sprinkle the salt all over the lamb leg.
Using a sharp knife, make diagonal slits across the top about ½ inch deep, then score in the other direction in a crosshatch pattern.
Divide the rosemary sprigs into tufts of three or four leaves. At the intersection of each crosshatch, insert rosemary sprigs and a thin slice of garlic.
Spread the anchovy butter over the top.
Place the prepared lamb leg into the Instant Pot and pour in the water or chicken stock.
Pressure Cook on High for 30 Minutes. Allow the pot to naturally release for 10 minutes, then release any remaining steam.
Discard any broth or juices collected in the pot. Return the lamb to the pot and insert the air fryer lid. Air Fry at 400°F for 5 Minutes, or until the lamb gets a nice golden color.
Allow the lamb to rest for at least 15 minutes before carving.
Serve with steamed golden potatoes, mash, or greens. Enjoy!