TO MAKE THE DAL:
Rinse lentils 2 or 3 times until water is not foamy and runs clear. In a large stockpot, combine lentils and water and all spices; bring to a boil. Reduce heat and simmer for at least 30 minutes (without lid) until the lentils are soft, somewhere between thick and soupy.
TO MAKE THE TADKA:
Heat oil in a small frying pan. Add the cumin seeds and let them sizzle for about 10 seconds. Add the red onion and saute until softened. Add the tomato and continue to saute until everything becomes close to caramelized.
Add tadka to cooked dal (lentils) and serve immediately with basmati rice.