How to Make It
In a large skillet, melt 2 tablespoons of the butter. Add the onion and garlic, season generously with salt and cook over moderately high heat, stirring occasionally, until the onion has softened and is starting to brown, about 8 minutes. Stir in 1/4 cup of the harissa and the tomato paste and cook, stirring, until fragrant, about 1 minute. Stir in the cumin and paprika, then add 2 1/2 cups of water and bring to a simmer, scraping up any browned bits on the bottom of the pan. Transfer to a 6-quart slow cooker along with the dried beans, carrots, thyme sprigs, bay leaves, lemon zest and lemon juice. Add 1 tablespoon of salt and 3 1/2 cups of water. Cover and cook on high until the beans are tender, about 6 hours.
Meanwhile, in a small nonstick skillet, melt the remaining 1 tablespoon of butter. Add the turmeric and cook over moderately low heat, stirring constantly, until it dissolves, about 30 seconds. Transfer to a medium bowl and let cool slightly, about 5 minutes. Slowly whisk in the yogurt until smooth. Season with salt.
Discard the bay leaves and thyme sprigs from the stew. Just before serving, stir in the thyme leaves, chopped parsley and the remaining 3 tablespoons of harissa; season with salt. Ladle into bowls and garnish with the preserved lemon and parsley leaves. Serve, passing the turmeric yogurt at the table.