Take small portions of the dough and place them on a rolling surface. Lightly grease the surface with a few drops of oil, then roll each dough ball into a 2.5-inch circle using a rolling pin. Don't dust with dry flour, use little oil as needed.
Add about 1 tablespoon of the dal filling to the center of the dough circle. Gently bring the edges of the dough together, folding them over the dal filling to seal it completely.
If needed, apply a few more drops of oil and gently press the filled dough to flatten it into a round shape. Roll gently with the pin to a 3 inch circle taking care not to apply too much pressure. Don't try to roll very thin else dal will start coming out. Repeat this process for all the kachoris, keep the rolled ones covered with a cloth to prevent drying out.
Heat oil in a deep pot over low-medium heat for frying. Tip:- The oil should be hot but not smoking. To check, drop a small piece of dough into the oil; if it sizzles and rises to top without changing color, the oil is ready for frying.
Carefully lower the rolled dal kachoris into the hot oil, one at a time. Fry them until they turn golden-brown and crispy, flipping and gently pressing them with a slotted spoon occasionally for even cooking. The heat might cause few kachoris to puff up. All of them will not puff up and that's how they are meant to be.
Once done, use a slotted spoon to lift the kachoris from the oil and drain excess oil. Fry the remaining kachoris in the same way, then transfer them onto paper towels to absorb any extra oil. Serve warm with curry and raita.