Prepare to use oven middle rack, preheat oven to 400 F.
Briefly rinse off leg of lamb and pat dry with clean paper towel. Squeeze a lemon half over each side of lamb, rubbing in the juice. Place in un-oiled pot. Pat the lamb with the garlic and garam masala. Season with the salt and pepper.
Place in oven, uncovered, for 30 min. Do not open oven. Reduce heat to 350 F for another hour for medium-rare. I recommend an hour and a half at 350, then turn off oven. Let lamb sit in juices untouched for 10-15min. Serve.