Add lentils and turmeric to water in pot. Stir and then bring to boil for 20 minutes until lentils are soft. Add tomatoes and salt and cook a few minutes longer, then reduce heat and simmer.
Heat oil in skillet, then add cumin and mustard seeds for a few minutes until fragrant. Add jalapeno, onion, and garlic and saute for 10 minutes until golden brown.
Add onion mixture to lentils and simmer a few more minutes. When ready to serve add fresh cilantro and remove from heat with lid on for a few minutes.