Place toor dahl in a mesh colander and rinse in warm water. Transfer to a saucepan. Cover with 2 cups water and salt; bring to a boil. Cook, covered, over medium heat, until tender, about 40 minutes.
Heat olive oil in a skillet. Add cumin; cook until cumin swells and turns golden brown, 1 to 2 minutes. Add onion, ginger, and turmeric; cook and stir until onion is softened, about 5 minutes. Add tomatoes, cilantro, and green chile; cook and stir until softened, about 10 minutes. Stir in the dhal. Simmer, covered, until flavors combine, about 10 minutes. Stir in lemon juice.