Roast the Garlic: Preheat your oven to 400°F (200°C). Slice the top quarter-inch off the head of garlic to expose the cloves. Place it on a small piece of aluminum foil, drizzle with 1/2 tablespoon of olive oil, and wrap loosely. Roast for 30-40 minutes, until the cloves are completely soft and caramelized. Set aside to cool.
Par-Cook the Lentils: While the garlic roasts, place the rinsed lentils in a saucepan and cover with 2 inches of water. Bring to a boil and cook for 10 minutes to ensure a creamier final texture. Drain the lentils thoroughly and set aside.
Build the Aromatic Base: In a large Dutch oven or heavy-bottomed pot, heat the remaining 3 tablespoons of olive oil over medium heat. Add the leek, carrot, celery, and fennel. Sauté for 8-10 minutes, stirring occasionally, until the vegetables have softened significantly and become deeply fragrant.
Bloom the Spices: Add the tomato paste, dried oregano, smoked paprika, and cumin to the pot. Stir constantly for 1 minute until the paste darkens slightly. This crucial step toasts the spices and deepens the flavor.
Simmer the Soup: Add the drained lentils to the pot, stirring to coat them in the spice and vegetable mixture. Pour in the low-sodium vegetable broth and add the bay leaves, sea salt, and black pepper. Bring the soup to a boil, then reduce the heat to low, cover, and let it simmer gently for 40-50 minutes, or until the lentils are tender.
Finish and Serve: Remove the pot from the heat. Squeeze the soft, roasted garlic cloves from their skins directly into the soup and stir vigorously until the garlic dissolves. Remove the bay leaves. Stir in the fresh baby spinach until it wilts, followed by the fresh lemon juice. Ladle the soup into bowls and garnish generously with fresh parsley or dill and a final drizzle of olive oil.