Take a pressure cooker; add 2 cups of water and salt. Heat them in high flame.
Once water start boiling, add the soaked horse gram, keep the lid slightly open and cook for 20 minutes in medium flame (do not pressure cook them, as they may get over cooked and will not be perfect to make sundal). Check whether they are cooked.
Once they are cooked, drain them with a colander, and keep aside.
Heat oil in a pan. Add mustard, urad dal and fry them, until they splutter.
Now add the nicely chopped onions and stir them, until they turn translucent.
Cut the green chillies into small round pieces and add them to the well-cooked onions, followed by grated carrots, curry leaves, fennel seeds and ginger.
Smash the garlic pods and add them too. Stir everything well and let them cook, for a while.
Once they are done, add the already cooked horse gram and mix everything well.
Add the grated coconut and grated mango. Stir them well, close the lid and put the flame in simmer.