Prepare the marinade for the lamb: Put the cumin and coriander seeds in a small, dry skillet set over medium heat. Cook until seeds are just toasted and fragrant. Remove. Put the toasted seeds into the chopping cup with the grinding blade along with the salt, pepper, garlic, oregano and cayenne. Pulse a few times to break up, scraping down as needed. Then add ¼ cup of the olive oil and process until a paste has formed.
Rub the paste all over the lamb (if it was tied or netted, be sure to remove string or netting so the lamb can be evenly coated). Transfer to a large mixing bowl, cover with plastic wrap, or place lamb in a 2-gallon re-sealable bag. Marinate in the refrigerator overnight.
Preheat oven to 400°F, with rack in the middle of the oven. Remove lamb from bowl/bag and tie well with butcher’s twine. Cover a baking sheet 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 tied, marinated lamb on top of potatoes and onions, centered in the pan.
Put the lamb in the oven. Roast for about 50 minutes, until the internal temperature reaches 130°F, for medium rare. Allow it to rest for
Transfer lamb to a stable cutting board. Using the knife attachment on Speed 5, slice the lamb thinly and serve with the roasted potatoes, onions, Yogurt Tahini Sauce and toasted pita bread.