Saute onion until soft, then add jalapeno, garlic and ginger for another minute or two.
Add lentils, water, tomatoes with juice and salt. Bring to a boil, then simmer until lentils are soft, 30-40 minutes.
Mash or puree lentil mixture, then keep warm.
Heat a small saute pan on medium-high, add oil, then heat cumin and mustard seeds for 15 seconds. Add turmeric and paprika for another 30 seconds. Add spices to lentil, warm mixture and serve.