Heat the oil in a large saucepan or Dutch oven on medium. Add onion and cook, stirring occasionally, until tender (they should not brown), 5 to 7 minutes. Stir in curry powder and cumin seeds and cook for 1 minute.
Stir in the lentils, then add ½ cup of water and the tomatoes (with their juices), coconut milk, red pepper flakes (if using), and 1 ½ teaspoons salt. Bring to a simmer, then cover and gently simmer, stirring occasionally, until the lentils are tender, 35 to 45 minutes, adding water if it starts to look dry.
While the lentils cook, in a small bowl, combine yogurt, ginger and pinch salt.
Serve the lentils with a hefty dollop of the gingery yogurt. You could also serve this over rice or throw in some greens, such as spinach or arugula.