Set a large pot to medium heat and add oil. Add the cumin and mustard seeds and saute until they begin to pop, then quickly add onion, ginger, turmeric, ground cumin, and a pinch of salt. Saute until onion begins to soften, about 3 min.
Add the tomatoes and saute for 2 more minutes. Pour in ½ cup of broth and reduce by half.
Add the red lentils and stir, then add remaining broth, and cinnamon stick. Bring to a boíl, then turn heat down to a simmer, cover and let cook for 30 min. or until tender.
Add salt and let simmer for another 5 min. Remove the cinnamon stick and pour in lime juice.
In a blender, puree the soup until very smooth. Adjust consistency according to preference with more broth.
Return soup to pot and reheat. Serve with chopped cilantro.