For the grilled lamb: Trim the excess fat and any gristle from both sides of the lamb and lay it flat on a cutting board. To make the lamb generally the same thickness, butterfly the thick ends by slicing shallowly on the diagonal into the thickest part and opening it up, like a book. Score the entire roast, both sides, in a shallow 1-inch-wide crosshatch pattern.
Heat a small saute pan over medium-low heat and add the coriander and cumin seeds. Toast until fragrant. Combine the toasted seeds with the sesame seeds and grind finely in a spice grinder or a mortar and pestle. Pour the ground spices into a bowl and add the sumac, salt and cinnamon.
Combine the garlic with a little bit of canola oil to make a paste, and rub into the meat. Rub the spice mix evenly into the meat and leave to penetrate for at least 1 hour at room temperature, or up to 1 day refrigerated.
For the herbed yogurt: Combine the yogurt, chives, cilantro, dill, olive oil, lemon juice, salt and cayenne in a bowl and set aside.
Heat a grill to medium-high heat.
Grill the lamb over medium-high heat, watching carefully for flare-ups, until dark brown on the underside, about 5 minutes. Flip and cook until dark brown again, about 5 minutes. Continue to cook the lamb, flipping every minute or so, until the lamb feels bouncy when poked and measures 125 degrees F on an internal-read thermometer for medium-rare, about 5 minutes longer. (If you like your lamb medium, bring it to 130 degrees.)
Let the meat rest for at least 5 minutes on a platter, flipping it occasionally, before slicing thinly across the grain and serving with the herbed yogurt.