Clean the mangoes and with a peeler remove the skin. Pat dry with a kitchen towel and using the same peeler, gently work around the flesh of the raw green mango or you can chop them into cubes as well. Do whichever is easier for you.
Pressure cook the mangoes and green chillie with 1/4 water for 3-4 whistles. Remove excess water. Mash them to a fine paste and cool it.
In the mean time, heat about 100 ml water in a cooking pan and throw in the jaggery and salt when the water is boiling hot. Maintain the heat at medium high. Let the jaggery dissolve and as they turn in to a thick syrup, which will happen in 15-20 minutes, add the mashed paste and give it a complete mix. Allow the mixture to thicken for 2-3 minutes and remove from flame.
In a small pan, heat the oil. Once heated, let the mustards splutter and add the urad dhal and cook for 2 minutes. Add this to the mango paste and serve hot.