Wash masoor dal thoroughly under running water and discard the water if any.
Take a pressure cooker and add the dal to it. Peel the green mango and cut it into long quarters through the stone. Discard the stone and add the mango to the dal.
Add turmeric powder, salt and enough water to cover the dal. Close the lid of the pressure cooker and cook the dal for a maximum of 2 whistles or 15 minutes. In case you don't have a pressure cooker, cook the dal in a saucepan with water till the dal and the mangoes soften.
Take a kadai and heat mustard oil. When the oil starts to smoke, add dried red chillies (you can break them and add too for greater flavour and heat) and mustard seeds.
Watch the mustard seeds dance in the oil and then add the cooked dal along with the mangoes and the water from the pressure cooker.
Give it a good stir and then add one more cup of water. If you want a thinner consistency, you can add more water.
Keep boiling and then add salt if needed. Add sugar to taste and balance the sourness of the mango.
Once the sugar has incorporated, take tok dal off the heat. Serve it with steamed rice.