Heat 1 teaspoon oil in a large heavy cast-iron skillet over medium-high heat. Add onion to pan; cook 2 minutes or until charred. Carefully turn over onion, and cook an additional 4 minutes or until blackened and charred. Remove from heat. Coarsely chop; set aside.
Combine mustard seeds, coriander seeds, cumin seeds, and clove in a small skillet over medium heat. Cook 1 1/2 minutes or until fragrant, stirring frequently. Remove from heat. Combine mustard mixture, cinnamon, cardamom, and chile in spice or coffee grinder. Pulse until ground.
Heat remaining 2 teaspoons oil in a small Dutch oven over medium-high heat. Add ginger and garlic to pan; sauté 1 minute. Stir in spices; sauté 1 minute. Add broth, lentils, and tomato to pan; bring to a boil. Cover, reduce heat, and simmer 30 minutes, stirring occasionally. Uncover; add onion, and cook 10 minutes. Stir in cilantro and juice.
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