Measure the vegetable broth, lentils, onion, carrots, preserved lemon and garlic into a slow cooker pot. Stir together, cover, and set on low for 8-10 hours. At this point, the lentils should be very soft. Mash some with a fork against the side of the pot to thicken.
Stir in the spinach and cilantro. Cover and continue to cook for 20 minutes longer. Salt and pepper to taste. Serve hot with fresh pita bread.