Wipe both sides of lamb with damp paper towels; trim off fat. To make lamb as even as possible, place large pieces of wax paper under and over butterflied lamb. Use a meat mallet or a heavy fry pan to flatten lamb as evenly as possible. (If ordering lamb from a butcher, ask him to make the meat even and remove the fat).
Using a whisk or fork, combine remaining ingredients in a 2-cup measuring cup. Place lamb in an extra large plastic bag. Pour marinade over meat and seal securely. Turn bag to evenly coat meat with marinade. Place closed bag on a baking sheet and refrigerate 8 hours or overnight, turning bag at least 4 times to evenly marinate lamb.
Remove lamb from refrigerator; let stand at room temperature for no more than 1 hour. Spray grill or broiler pan with cooking spray; preheat grill or broiler.
Reserve marinade for basting and place lamb on grill or broiler pan; baste with marinade. Depending on size of meat, cook lamb for about 50 minutes, basting and turning meat every 15 minutes. Lamb is cooked when instant meat thermometer reaches 145°F for medium, or 165°F for well done. Do not overcook!
Remove meat from grill or boiler; cover with aluminum foil. Let meat rest about 15 minutes for easier slicing.