Step 1Boil the potatoes in salted water until tender, about 20 minutes. Drain them, reserving the water. Coarsely mash the potatoes.
Step 2Heat the oil in a nonstick saucepan over medium-high heat. Keep all the ingredients close at hand. When the oil reaches the smoking point, add the mustard seeds. These will immediately start to splutter and pop. Keep a splatter screen handy to fend off any hot oil burns. When the spluttering eases off, about 5 to 8 seconds, reduce the heat and add the dal and the curry leaves. Fry a few more seconds until the dal turns brown.
Step 3Add the onions and fry until golden brown, 12 to 15 minutes. Then add the potatoes, turmeric and salt. Mix well. Add 3 to 4 tablespoons of the reserved potato water, mix again and cook until the water is absorbed and the bhaji is fairly dry in texture, 2 to 3 minutes.
Step 4Cook the Sada Dosa (following the Sada Dosa recipe directions), and as each dosa is finished, spoon about 2 tablespoons of the bhaji onto one half of the dosa, then fold the other half over it. Restaurants often place the bhaji in the center of the dosa and fold the two sides over it to make it rectangular rather than half moon shape. Repeat until all the filling is used.