Start by soaking the moong and masoor daal in warm water for 15-20 minutes. Set aside, while you make the masala for the dal.
In a large pot, heat the oil and fry the onion for 3-4 minutes until it is softened and translucent.
Then add the tomatoes and the spices and a splash of water.
Cook on medium heat until the tomatoes have softened and the oil has come to the surface of the masala. This will take about 5-7 minutes, and be sure to stir frequently.
Now drain the soaked dal, and add it to the masala. Add 1 cup of water and on medium heat cook the dal for 25-30 minutes, or until it has cooked.
You may need to add more water depending on how thick the dal is and what kind of consistency you like.
Once the dal is cooked, set the pot aside.
For the Tadka
In a frying pan, heat the oil and fry the sliced onion until it is golden. Add the garlic and fry for about 30 seconds. Now add the green chilli and fry for a 30 seconds or so, and finally add the cumin seeds which only take 10 seconds or so to release their aroma.
Carefully pour this tadka (tempering) onto the cooked dal and top with chopped coriander.