Mix the olive oil, salt, pepper, garlic powder, dried rosemary, and ground cumin in a small bowl.
Rub the olive oil mixture all over the meat. Place the lamb, flat side down and fat cap up, in a rimmed roasting pan on a greased rack. Allow it to rest on the counter at room temperature for one hour.
Preheat your oven to 425°F. Insert an oven-safe meat thermometer into the thickest part of the lamb without allowing it to touch the bone. Set the thermometer to 135°F (medium-rare). Roast the lamb for 15 minutes in the preheated 425°F oven.
Lower the oven temperature to 375°F and continue roasting until the thermometer alerts you that the meat has reached an internal temperature of 135°F. This should take about 1 hour and 20 minutes (about 20 minutes per pound).
Remove the lamb from the oven, cover it loosely with foil, and allow it to rest for 20 minutes before slicing and serving.
Video
Notes
If your lamb is bigger than 4 pounds, it will need to spend more time in the 375°F oven - as mentioned above, about 20 minutes per pound after the initial 15 minutes at 425°F. If this is the case, it's a good idea to loosely cover it with foil after it has spent an hour and a half in the oven. Otherwise, the top could get too dark.
If the roast is significantly larger than 4 pounds, increase the olive oil and spices accordingly. So, for example, if your lamb is 6 pounds, increase them by 50%.
The USDA says you should cook roasts to medium (145°F).
You can keep the leftovers in the fridge, in a sealed container, for up to 4 days. They are excellent when gently reheated in the microwave, covered, at 50% power. But don't overcook them, or they'll become dry. I also enjoy eating cold slices of the leftovers with quick pickles, Dijon mustard, or sriracha mayo.