With a roast, like a butterflied leg of lamb or filet of beef, it is important to secure it with string before cooking. This makes a compact shape that cooks more evenly.
To tie a roast with individual knots, position it lengthwise seam side down. Put the string to one side. Run a length of string under the center of the roast.
Cross the two ends of the string over, just as if you were tying a shoe. Then pass the short length of string behind the longer one and loop it back into a knot. Pull the longer length and push slightly on the knot until its snug. Trim with scissors or a knife.
Continue to make knots on both sides, at about 2-inch intervals until the roast is secure.
If you want to tie a roast with a continual length of string, start by tying the string off about 2 inches from the end. Then drape the string over your hand in a loop -- make sure the tail-end of the string passes under the loop. Slip this over the end of the roast and position it about 2 inches from the first knot.
Pull the string until its snug but not digging into the meat. Position the string so each cross lines up. Repeat this down the length of the roast, making loops ever 2 to 3 inches.
At the end, flip the roast over. Cut the string, so it is as long as the roast. Loop the string under each crossing string and before tying it off at the end.

Instructor: Robert Bleifer