Whisk kefir, Ras el Hanout, garlic and salt together in a medium bowl. Ras el Hanout is a vibrant Moroccan spice blend made of ground cumin, chili peppers, paprika, turmeric, and baking spices like cinnamon, cardamom, clove, nutmeg, mace, allspice, and ginger. Find it in the spice aisle of your local grocer or gourmet store. Otherwise, it is easy to purchase online.
Place lamb in a large resealable plastic bag and pour in marinade. Seal bag, pressing out any air. Chill at least 3 hours and up to 24 hours.
Prepare the grill for direct cooking over high heat (450° to 550°F); thoroughly clean grates and brush with oil.
Remove lamb from marinade shaking off any excess. Grill, turning once, until charred and cooked through, about 3 minutes per side for medium-rare. Transfer to a platter; let rest 10 minutes.
Grill spring onions, turning often, until lightly charred on all sides, about 4 minutes.
Top lamb with grilled spring onions and dust with Ras el Hanout.