Cut the fat off the lamb. You don’t need to be neurotic about it, just get the big parts off.
Blend the mustard, soy sauce, garlic, herbs, and ginger together in a bowl.
Beat in the olive oil by droplets to make a mayonnaise like cream.
Paint the lamb with the mixture and set it on the rack of the roasting pan.
The meat will pick up more flavor if it is coated several hours before roasting. (or overnight)
Roast in a 350°F (176°C) oven, for about 20 min. per pound if cold, to an internal temperature of 135° to 140°, then let it sit for about 20 min. The internal temperature will rise in that time.
140° - 150°F (60° - 65°C) internal temperature is medium rare; figure 14 min. per pound if meat is cold; 10'- 12' if meat is room temperature. I like to let the meat soak up flavor overnight in the refrigerator. If you can poke holes in the leg to let the sauce get inside the meat, it helps a lot too. It is important to make sure the oven is up to temperature because an under-done leg of lamb isn't all that much fun. The lamb actually tastes better when it cooks, it picks up flavor. If there are people who like rare meat, there will be some, you can count on it. Don't forget to take off all the fat on the meat.
Note: Maybe increase the mix for large lamb legs.