Presoak the clay pot, top and bottom, in water for 15 minutes.
Make slits in the leg of lamb with a sharp knife and insert the slivers of garlic.
Mix the oregano, salt and pepper and rub over the lamb.
Insert a meat thermometer, away from the bone, and place the lamb in the pot.
Mix the wine and lemon juice and pour over the lamb.
Add the onions and green peppers.
Place the covered pot in a cold oven.
Set the temperature at 480 degrees.
After 60 minutes, check the meat thermometer and cook according to taste, well done, medium or rare.
When the meat is done, pour off the liquid into a saucepan, bring to a boil and thicken with the arrowroot. (If lamb is insufficiently browned, remove the lid from the pot for the last ten minutes of cooking.)