Masala dosas -- large, thin, savory crepes typical of South Indian cuisine -- are easily one of my top five foods, and it's all because of the filling: spicy, hangover-level comforting, semi-mashed potatoes. Dosas are often enormous and very difficult (for me, at least) to finish in one sitting. But I'll always polish off the potatoes. So my mom thought, "Why not make just the best part of the dosa?" Her version is sort of traditional (we're not South Indian), but with the worthy additions of lime and squiggles of ketchup -- the former to cut through the richness of the potatoes, the latter because . . . potatoes and ketchup, of course!
Using your hands, break the potatoes into 1/2-inch pieces and set aside.
In a large skillet over medium-high heat, warm the oil. Once the oil begins to shimmer, add the black mustard seeds and as soon as they begin to pop and dance around in the oil, which should be within seconds, remove the pan from the heat. Add the curry leaves, making sure they get fully coated in oil (there may be more popping and splattering, and that's okay!). The leaves should immediately crisp up in the residual heat. Add the turmeric, asafetida (if using), and onion, return the pan to medium-high heat, and cook, stirring, until the onion starts to become translucent, 2 to 3 minutes.
Add the potato pieces, salt, red chiles, and chile powder, followed by 1/4 cup water. Cover and cook until the potatoes are soft and slightly mushy but still retain some of their shape, 4 to 5 minutes.
Serve the potatoes with lots of lime wedges and ketchup to go on top.