This is an easy recipe. I used to use Trader Joe's frozen boneless leg of lamb, but that item has gone the way of the typewriter.
The hardest part of this process is tying the roast. Please try to remember that the purpose of tying the roast is to make it uniform in size, not to win a merit badge. A simple package tie can often work.
Unravel the leg of lamb. Place on a work surface with the fat side touching the work surface, the inside facing you.
Spread 2 tablespoons of the Dijon mustard on the inside as evenly as possible.
Sprinkle the slice garlic over the mustard.
Season with salt and pepper.
Roll the leg of lamb so it looks like a roast. Take the string and wrap the roast as if it were a gift and the twine is the ribbon--once round the width, cross and once around the length, tie a square knot. If you are nervous that the roast will unravel, tie a series of knots across the width and down the length of the roast.
Place lamb in roasting pan seam side down. Spread remaining tablespoon of mustard on top of roast and season with salt and pepper. Insert a meat thermometer in roast.
Place in 400 degree oven for 20 minutes reduce heat and continue cooking for about an hour or until that thermometer reads the doneness you like.