Chop the amaranth leaves finely and place in a large pot or in a pressure cooker along with the mung dal and turmeric. Add enough water to cover by about an inch. Bring to a boil, cover, and cook until tender, adding more water if needed. Or if using pressure cooker, pressure-cook for the recommended amount of time for mung dal.
Heat the oil. When it shimmers, add the cumin seeds, stir for a few seconds, then add the garlic.
Saute for 30 seconds, then add the tomatoes. Let the tomatoes cook down until they are pulpy.
Add the cumin and coriander powders, aamchur, and red pepper flakes or chilli powder.
Stir everything well and let it all cook for a couple of minutes.
Add the dal along with the amaranth leaves. Bring to a boil and lower heat to a simmer. Add water if the dal is too thick. It really depends on your taste-- I like my dal slightly runny.
Let the dal simmer for about 5 minutes, then turn off heat and serve hot with rice or rotis.