1. Prepare the marinade for the lamb: Put the cumin and coriander seeds in a small, dry skillet over medium heat. Cook until seeds are just toasted and fragrant. Put the toasted seeds in a Cuisinart® spice grinder or chopper along with the cinnamon sticks, salt, pepper, garlic, oregano and cayenne. Pulse a few times to break up, scraping down the sides as needed. Add ¼ cup oil and process until a paste has formed.
2. Rub the paste all over the lamb (remove any butcher’s twine or netting so the lamb can be evenly coated). Transfer to a large mixing bowl, cover with plastic, or into a 2-gallon resealable bag. Marinate in the refrigerator overnight.
3. Once lamb has marinated, remove from bowl/bag and tie tightly with butcher’s twine in 2-inch intervals. Cover the baking pan with foil. Add the potatoes and onions, and toss with the remaining olive oil and a couple pinches of salt and a pinch of pepper. Place the lamb on top, centered in the pan.
4. Adjust the rack to position 1 and put the lamb in the oven. Select Dual Cook, first set to Roast at 400°F for 30 minutes, then set to Roast at 300°F for 30 minutes. Press Speed Convection, then Start, to begin roasting.
5. The lamb is cooked when the internal temperature reaches 130°F, for medium rare. Allow it to rest for 10 minutes before slicing and serving.
6. While lamb is cooking, prepare the yogurt sauce. In a small bowl whisk together all of the sauce ingredients until homogenous. Reserve in the refrigerator (this can be made in advance and be stored in an airtight container in the refrigerator for up to 1 week).
7. Slice lamb thinly and serve with the roasted potatoes and onions, along with the yogurt sauce and toasted pita bread.