Rinse and dry lamb.
In a blender, combine olive oil, shallots, garlic, thyme leaves, lemon peel and juice, mustard, cumin, allspice, cinnamon, and cayenne; whirl until coarsely puréed. Brush 1/4 cup spiced oil all over lamb; save remaining for grilled ratatouille.
Prepare barbecue for indirect heat.
If using charcoal briquets, mound and ignite 60 briquets on the firegrate of a barbecue with a lid. When briquets are dotted with gray ash, in 15 to 20 minutes, push equal amounts to opposite sides of firegrate. Add 5 more briquets to each mound of coals now and after 30 minutes, if grilling takes longer than that.
If using a gas barbecue, turn all the burners to high, close lid, and heat for 10 minutes. Then adjust burners for indirect cooking (heat on sides of grill not down center under food) and keep on high.
Set a drip pan on firegrate between coals or burners. Set barbecue grill in place. Set roast in center of grill, not over heat. Cover barbecue; open vents for charcoal.
Cook lamb loin until a thermometer inserted in center of thickest part reaches 135° for medium-rare, 25 to 30 minutes, or until it's as done as you like.
Transfer roast to a platter and, keeping it warm, let rest 5 to 10 minutes. Surround with grilled ratatouille, and garnish with thyme sprigs if desired. Cut roast into thin slices and add salt and pepper to taste.