If you are not able to grill the lamb, simply marinate flat, without butterflying, truss/tie and roast.
Prep Time:24 hours
Cook Time:30 minutes
Total Time:24 hours 30 minutes
1/2 cup extra-virgin olive oil
1/4 cup fresh lemon juice
8 garlic cloves, minced
2 T Dijon mustard
1 tablespoon dried oregano (Greek or Mediterranean), crumbled to release oils (Greek seasoning containing, oregano, mint and dried onion can be used in place of the oregano alone)
2 teaspoons salt
1 teaspoon black pepper
1 Onion, peeled and cut in half
1–4 1/2 to 5-lb butterflied boneless leg of lamb, trimmed of fat
2 lemons cut into four wedges each
Combine oil, lemon juice, mustard, garlic, oregano, salt, and pepper in a seal-able plastic bag. Add lamb and seal bag, pressing out air. Turn bag to coat lamb, then put bag in a shallow baking pan and marinate, chilled, turning bag over occasionally, at least 8 hours or overnight. The lamb will slightly change color as the marinade penetrates the meat.
About 1 hour, before grilling, remove lamb from marinade (discard marinade). Bring lamb to room temperature. Salt and Pepper the lamb on both sides.
Prepare grill for cooking over direct heat with medium-hot charcoal (moderate heat for gas). Take half an onion and dip it in vegetable or other high-heat oil. Rub the onion over the entire surface of the hot grill. This will oil the grill and also add flavor.
Grill turning over occasionally and, if necessary, moving around on grill to avoid flare-ups, until thermometer registers 125 to 128°F, 10 to 15 minutes total for medium-rare or refer to the guide for desired temperature.
Grill lemon wedges during the last few minutes while cooking the lamb, until lightly charred on both sides.
Transfer lamb to a cutting board. Let lamb stand, loosely covered with foil, 20 minutes. (Internal temperature will rise to 135°F while meat stands.) Cut across the grain into slices.