Rinse the lentils in a fine-mesh sieve under cold water until the water runs clear, then drain well.
In a large pot, combine the drained lentils and 6 cups water and bring to a boil over medium-high heat.
Reduce heat and simmer the lentils until very soft and breaking apart, about 10 to 15 minutes, skimming any foam; drain well.
While the lentils cook, heat the oil in a large saucepan over medium-high heat until shimmering.
Add the mustard seeds, cumin seeds, and chopped onion; cook, stirring, until the seeds crack and the onion softens, about 5 to 8 minutes.
Stir in the grated ginger and minced green chili and cook until fragrant, about 1 minute.
Add the canned whole tomatoes with their juices and mash them to break up the pieces; cook until some of the tomato liquid evaporates, about 10 minutes.
Stir in the red chili powder, ground coriander, turmeric, and salt and cook for 2 minutes to bloom the spices.
Add the drained cooked lentils to the tomato-spice mixture and stir to combine; cook briefly to meld flavors, adjusting consistency with a little water if desired.
Garnish with fresh cilantro leaves and serve hot with rice, yogurt, and naan.