1 Make the marinade: Put onion, garlic, rosemary, lemon zest, apple cider vinegar, and olive oil into a food processor and pulse to combine. (If you don’t have a food processor, just chop the onions, garlic, and rosemary very well and combine with the rest.)
2 Marinate the lamb: Sprinkle a generous amount of salt and pepper over the lamb. Place marinade and lamb into a 1-gallon freezer bag. Spread marinade over all sides of the meat. Seal the bag and refrigerate for 1 to 2 hours.
3 Remove lamb from refrigerator and let come to room temperature (about 20 minutes). When ready to put on grill, remove from marinade bag.
To help make the lamb easier to turn on the grill you can insert a couple of skewers through the lamb, crosswise. (A tip learned from Rick Rodgers in Kingsford Complete Grilling Cookbook.)
4 Prepare grill: If you are using a charcoal grill, prepare the coals so that they are double layered on one side of the grill, and sparsely single layered on the other side of the grill (this is called “banked” grilling).
If you are using a gas grill, heat the grill on high on all burners to start. After the initial browning you will reduce the heat.
5 Sear the lamb: Place the lamb, fat side down, on the grill on the hot side (double layer charcoals). You will get likely get flareups, so be prepared with a squirt bottle of water or a couple of cups of water (if using a charcoal grill) to control the flames if needed. (My brother Matt swears by shaking the bottle of beer he is drinking to squirt some beer on the coals when needed for flareups.)
Sear one side for 4 minutes, then flip the lamb over to sear the other side for another 4 minutes.
6 Move to cool side of grill and finish cooking: If you are using a charcoal grill, move the roast to the less hot side of the grill. If you are using a gas grill, lower the heat to low. You will want to maintain a temperature of 300-350°F.
Cover the grill and let cook for an additional 35-45 minutes (depending on how thick, and how many pounds the roast is), until a meat thermometer inserted into the thickest part of the roast registers 130°F (for medium rare).
7 Let rest before cutting: Transfer to a cutting board with a well to catch the juices. Cover with aluminum foil and let rest for 5-10 minutes. Remove the skewers if you are using any.