Preheat oven to 450ºF and place the oven rack in the lower third of the oven.
In a 13×9-inch baking dish or roasting pan, toss the potatoes with the one tablespoon of the olive oil and season with a good pinch each of salt and pepper. Set aside.
In a small bowl, mix together the rosemary, thyme, creamy garlic sauce, basil pesto, salt, pepper and the remaining tablespoon of the olive oil to make a paste. Rub all over the lamb.
Tie the roast: the goal is to get the lamb roast to be a uniform shape and size (as much as possible) so that it cooks evenly. Start by placing the lamb fat side down on a cutting board with the short end closest to you. Wrap kitchen twine around the short side and tie in tight, firm knot. Tie 3 or 4 more just like this depending on the length of the roast. With one long length of kitchen string, wrap the roast lengthwise, tucking in the ends lightly and slipping the string under the prior ties. Knot off and trim neatly.
Place the lamb on top of potatoes, fat side up and rub any excess herb rub on top.
Roast until the internal temperature reaches 140ºF, about 45-60 minutes for medium rare/medium. After the first 30 minutes, check the temperature every 10-15 minutes until the internal temperature is reached. If the roast becomes too dark toward the end of roasting, tent with a piece of foil.
Once the lamb has reached the correct internal temperature, remove from the oven and rest for 15 minutes, tented with foil to keep warm.
Remove and discard the kitchen string, slice, and serve warm with the roasted potatoes.
Chef’s Tips
The resting time after roasting is important as it makes a big difference in how juicy and delicious the lamb will be. During cooking, all the juices inside the meat start to flow. When the lamb is removed from the oven, those juices need a chance to settle. If cut too early, the juices will flow out of the roast, resulting in a dry slices of meat.
Use a meat thermometer: while a boneless lamb roast of this size is typically done around 45-60 minutes in a 450º oven, results can vary. The best way to check is to use a meat thermometer, checking every 10-15 minutes after the first ½ hour until an internal temperature of 140ºF is reached for medium rare/medium.
Rare: 125ºF (52ºC)
Medium-Rare: 135ºF (57ºC)
Medium: 145ºF (63ºC)
Medium-Well: 150ºF (66ºC)
Well Done: 160ºF (71ºC)
Storage: cooked, roast lamb can be stored in the fridge in a sealed, covered container for up to 3 or 4 days. Make delicious sandwiches or soups with the leftovers.
