Place the lentils in a large saucepan or Dutch oven and add water to cover. Give the lentils a good rinse, stirring them with your fingertips; this will make the water cloudy. Drain the water (I tilt the pan over the sink to drain it). Repeat this once or twice. Then pour in 4 cups of water and add the potato, onion, chiles, salt, and the whole tomato.
Bring the water to a rolling boil over medium-high heat. Fish the whole tomato out of the pot; it will now look shriveled with loose skin. Transfer the tomato to a bowl.
Reduce the heat to medium and cover the pan. Let the lentil medley simmer, stirring occasionally, until the potato is fork-tender and the lentils are now yellow, about 15 minutes. Even though the lentils start with a beautiful salmon color, cooking them yields a yellow color (as the Indian bobble-head doll said, “Eh, it is the will of Rama”). It’s the nature of these lentils and does not signify any wrongdoing on your part.
While the soup simmers, core the tomato and remove the loose skin, working over a bowl to catch the juices. As soon as the lentils are cooked, add the tomato, including any juices, to the pan. Pile in the spinach and cover the pan. Let the soup continue to simmer, no need to stir, until the spinach wilts, about 5 minutes.
Meanwhile, heat the oil in a small skillet over medium-high heat. Once the oil appears to shimmer, add the mustard seeds, cover the skillet, and cook until the seeds have stopped popping (not unlike popcorn), about 30 seconds. Turn off the heat and sprinkle in the cumin seeds, which will instantly sizzle, turn reddish brown, and smell incredibly nutty. Immediately transfer the mustard and cumin seed oil (seeds and all) to a small heatproof bowl (the longer it sits in that hot skillet, the more burnt the spices will become).
Once the spinach has wilted, ladle a third of the contents of the pan into a blender. Hold the lid down and run the blender in pulses to create a bright green puree. Pour the puree into a medium-size bowl. Repeat twice until all of the soup is pureed, adding the contents each time to the soup in the bowl. Alternatively, if you have an immersion (or stick) blender, you can puree all the soup in the pan.
Reheat the soup over low heat, if necessary, and stir in the spiced oil, including all of the seeds. Ladle the soup into individual bowls and serve warm.