For the ras el hanout marinade and lamb: Combine the canola oil, vinegar, cardamom, coriander, ginger, cinnamon, nutmeg, allspice, cloves, saffron, garlic, onions, 1 teaspoon salt, 1/2 teaspoon pepper and 1/4 cup water in a blender and blend until smooth.
Place the lamb in a dish large enough to hold the lamb and the marinade. Pour the marinade over and turn to coat. Cover and let marinate in the refrigerator for at least 4 hours and up to 24 hours.
For the red pepper-walnut tahini sauce: Combine the walnuts, tahini, yogurt, vinegar, cumin, cayenne, bell peppers and some salt and pepper in a food processor and process until smooth.
Remove the lamb from the refrigerator 30 minutes before grilling, and let sit at room temperature.
Heat a charcoal grill for indirect heat.
Remove the lamb from the marinade and pat dry. Sprinkle on both sides with salt and pepper. Grill until charred on both sides and cooked to medium (140 degrees F on an instant-read thermometer), about 30 minutes. Remove from the grill and let rest for 15 minutes.
Slice into paper-thin slices across the grain. Place several slices of the meat in each pita and drizzle with the red pepper sauce.