Place lentils in a saucepan and cover with 2 cups hot water; set aside to soak, about 30 minutes.
Bring lentils to a boil and add turmeric. Cook, skimming the surface, until lentils are soft, about 30 minutes. Drain and let cool.
Heat 2 tablespoons olive oil in a large skillet over medium heat. Add onion; cook and stir until golden brown, about 5 minutes. Add garlic, cumin, cardamom, and ginger. Cook until fragrant, 2 to 3 minutes. Remove from heat and let cool, about 10 minutes.
Fold onion mixture into lentils along with remaining 2 tablespoons olive oil, cilantro, and lemon juice. Transfer to a blender and puree until creamy.