Bring 1/2 cup broth to a boil in a pot; reduce heat and simmer. Add onion and garlic and simmer until onion is translucent and tender, about 5 minutes. Stir coriander, cinnamon, turmeric, garam masala, and cayenne pepper into onion mixture; simmer for 1 minute.
Stir lentils into spiced onion mixture; cook, stirring constantly, for 30 seconds. Add remaining 3 1/2 cups broth and tomatoes; bring to a boil. Reduce heat to low, cover, and simmer until lentils are tender, 10 to 12 minutes. Stir lemon juice into soup and garnish with feta cheese.
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