Toast the coriander and the cumin in a saute pan over medium heat until the spices become fragrant and just begin to smoke, 2 minutes. Set the spices aside to cool and then blend them in a spice grinder.
Combine the yogurt, lime, cumin, coriander, paprika, ginger, cinnamon, turmeric, cayenne, sugar, salt, and pepper in a medium bowl.
Poke the lamb several times with a fork and place in a large resealable plastic bag with the garlic and the onions and pour in the yogurt mixture. Move the lamb around in the bag to coat completely and place in the refrigerator to marinate for 4 to 6 hours.
Remove the lamb and the onions from the marinade and let sit at room temperature for about 20 minutes. Discard the marinade. Heat the grill to high, sear all sides of the lamb and adjust grill to indirect heat, about 350 degrees F. Grill the lamb for about 45 minutes per side for medium. When the lamb is cooked, remove it to a platter to rest for 15 minutes before cutting.
Add the onions and the lime halves to the grill and cook until caramelized and tender, about 10 minutes. Remove the onions and the limes from the grill and set aside.
Bring a large pot of salted water to a boil over high heat, add the green beans and cook for 2 minutes. Remove from the pot to an ice bath to stop the cooking. Drain on a paper towels. Heat a large skillet over low heat and add the ghee and the green beans and allow to cook through. Squeeze the juice of 1/2 a grilled lime over the top, toss to coat and transfer to a platter. Garnish with the sesame seeds.
Slice the lamb and plate with the green beans and onions. Garnish with the remaining grilled limes.