The most difficult part of making Negima is said to be slicing the meat thin enough to wrap around the scallions. Worth asking a butcher for ultra-thin cut sirloin. maybe freezing the meat a bit first would make it easier to slice at home. Possibly easier is using pork, chicken, or veal sold as thin cutlets. With a little pounding they're thin enough. From Bittman's Best Recipes in the World.