Place lentils in a medium saucepan, cover with water by 2 inches, bring to a boil, then simmer 15 minutes. Drain.
Meanwhile, heat 2 tablespoons olive oil in a large Dutch oven over medium. Add half the leeks; cook 8–10 minutes until deeply golden and crisp. Transfer to a paper towel–lined plate, leaving oil in the pot.
Add remaining 1 tablespoon oil to the pot. Add remaining leeks; sauté 3–4 minutes until softened. Stir in garlic; cook 1 minute. Add cumin, coriander, allspice, cinnamon, turmeric, and smoked paprika; stir 30 seconds until fragrant.
Add rice; stir to coat. Add par-cooked lentils, 5½ cups broth, bay leaf, salt, and pepper; bring to a strong simmer.
Reduce to the lowest heat, cover tightly, and cook 35–40 minutes, until rice and lentils are tender and liquid is mostly absorbed. If liquid is absorbed early and grains are still firm, add hot water or broth ¼ cup at a time and continue cooking.
Turn off heat; let stand covered 10 minutes. Discard bay leaf, fluff gently, and adjust seasoning with additional salt if needed.
Divide among 8 bowls. Top with crispy leeks, sumac, and parsley. Serve with lemon wedges and (optional) yogurt.