Heat olive oil in a large Dutch oven over medium-high heat. Add the onion and leeks, sprinkle with salt and pepper, and cook until softened and golden, about 5 minutes. Add the carrot and bell pepper and cook for an additional 5 minutes, stirring occasionally, then add the garlic and cook for 1 minute more, or until the garlic is fragrant.
Add the ground lamb and cook until no longer pink, breaking up the meat as it browns, about 5 minutes. Add the hawaij, cumin, coriander, and nutmeg, then stir for 1 minute. Add the broth, lentils, sweet potatoes, and harissa paste with preserved lemon and stir until combined. Bring mixture to a low boil then reduce heat to a simmer and cook, covered, for 45 minutes, or until lentils are cooked and potatoes are softened.
Adjust seasoning as desired, then stir in spinach until wilted and serve with a dollop of Greek yogurt, red chiles, and cilantro.