Add moong dal to a bowl and rinse them thrice. Soak in 1¾ to 2 cups water until you chop and prepare the rest.
Heat oil in a cooker or pot and add cumin seeds. When the seeds splutter, add ginger, green chilies and 4 to 5 curry leaves. Fry until aromatic for about a minute and add onions if using. Saute until transparent for 2 to 3 mins.
Add tomatoes and salt. Saute for 1 to 2 mins. Stir in turmeric, red chilli powder and garam masala. Optionally saute for 1 to 2 mins.
Add the moong dal along with the soaked water. Mix well to deglaze the bottom of the cooker. Cover and pressure cook for 4 to 5 whistles on a medium flame.
When the pressure drops, open the lid and add kasuri methi and amchur. If you prefer you can mash the moong dal a bit. Taste test and add more salt if required.
Heat 1 tablespoon ghee in a small pan, on a medium flame. Add the crushed garlic and dried red chillies. Let the garlic fry a bit but not brown.
Turn down the heat to low and add mustard seeds, cumin seeds and curry leaves. Turn off the heat and stir in red chilli powder and hing.
Pour this over the moong dal and mix gently. When the dal cools down a bit squeeze some lemon juice and serve.
