2/3 of the thickness of a NY doesn't tell me much. A NY can be 1/2 an inch or 2 inches or more thick, so can sirloin. If it's 3/4 of an inch or more thick, you can cook it on the stovetop without ruining it. You can either cook it in a very heavy oiled pan, like cast iron, heated very hot, searing it on both sides and finishing it to your liking, or you can saute it it at somewhat lower heat in butter or olive oil or a combination of both. A stovetop grill or grill pan is a good option too. The key to top sirloin is the same as NY, don't overcook it.