In a medium roasting pan or Dutch oven, place boneless leg of lamb roast and make 20-30 inserts with a paring knife. Stick garlic slice into each opening until you run out of garlic.
Rub roast with oil all around. Then sprinkle one side with half of rosemary, salt and pepper. Turn around and repeat on another side.
Bake uncovered according until desired doneness. Remember meat will keep "cooking" as it rests.°Medium Rare: 130 - 135 degrees F internal temperature - about 16 minutes/lb.°Medium Well: 140 - 145 F internal temperature - about 18 minutes/lb.°Well-Done: 150 - 155 F internal temperature - about 20 minutes/lb.
My best advice for 4-5 lbs roast is to check internal temperature after 1 hour and go from there. For 2-3 lbs roast, check after 45 minutes. It all comes down to checking often after a certain period of time towards the end.
Remove lamb roast from the oven and let rest uncovered for 15 minutes.
Using a sharp knife, remove the casing and discard it.
Slice lamb into 1/4 inch slices against the grain. Serve warm at a celebration table or use cold leftovers in sandwiches or salads.
Roast Potatoes in Fat Drippings (Optional)
Scoop out 1 cup of fat and drippings from the pan and leave in the rest.
Add potatoes, carrots, garlic powder, salt and pepper and stir well.
Bake at 450 degrees F for about 35 minutes or until fork tender. You can prep the veggies and seasonings while lamb is cooking, so after just pop them in without even turning off the oven.
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Notes
Make ahead: To save time during a busy day of cooking of a holiday, you can stuff and season roast, cover and refrigerate for 24 hours. Then let it come to room temperature and roast as per recipe.
Store: Refrigerate roast whole or sliced in an airtight container for up to 5 days. It tastes so good cold.
Freeze: Freeze leftovers sliced or cut into bite sized pieces in an airtight container for up to 3 months. Thaw in refrigerator.
Don’t remove the casing before cooking: Under any circumstances keep netting or rope on. It is what is holding pieces of meat together. If you remove it, meat will fall apart.
Adjust for your oven: All ovens vary. Rule of thumb is to start checking with internal meat thermometer 1/2 hour before end of cook time as per recipe.
Meat will keep "cooking" as it rests: Please keep in mind internal temperature will rise while meat is resting after baking in the oven.