1. Heat olive oil in a tagine or large pot over medium heat.
2. Add onions and garlic, and sauté until softened.
3. Add potatoes, preserved lemon (rinse before using), olives, vegetable broth, and spices.
4. Cover and simmer for about 30 minutes, or until the potatoes are tender.
5. Garnish with fresh herbs and serve with couscous or crusty bread.