Rinse the mix of dals 3-4 times in running water until the water runs clear. Washing lentils is key in removing the indigestible elements found in their skins. Soak the lentils for 20-25 minutes. Soaked lentils cook evenly.
Place the soaked & drained lentils in a pressure cooker. Add 3 cups fresh water to cover. Set on a stove. You could cook the lentils in Instant pot as well. Once boiling, you will notice white foam on the surface of lentils. Using a spoon, skim the foam and discard it.
Top with ½ cup more water to cover the lentils. Add turmeric powder and salt. Close the lid and cook on low medium heat for 3 whistles. The lentils should be soft in 3 whistles. We dont want mushy lentils.
Once the pressure is released, open the lid and make sure that lentils are soft. I press a few grains between my thumb & index finger to check it. Else cook for 1-2 whistles more or as needed. Mash the cooked lentils using back of spoon for a few minutes till creamy.
While the lentils are cooking, clean the methi leaves. To clean, pluck the leaves from the stems and discard the stems. Thoroughly wash the methi leaves 3-4 times and finely chop them. Chop the garlic and ginger as well. In a pot, warm up the mustard oil to slightly smoky (to do away the raw smell).
Take off the stove and temper with cumin, hing, chilies and chopped garlic & ginger. Everything will sizzle, keep stirring and make sure that nothing burns. Sauté exerting in oil for a few seconds.
Next, add the chopped methi leaves and saute for 3-4 minutes to do away the bitterness of greens. This also dries up the moisture of greens and prevents the dal from becoming greenish color.
Add the cooked dal and tomato (if using). Mix, adjust the consistency with extra water if needed and simmer for 8-10 minutes till everything is warmed through.
Finish with amchoor (or lemon juice). You could also add a few pinches of garam masala. Done!