Put the potatoes in a medium bowl, cover with cool water, and set aside.

Heat the canola oil and the mustard seeds in a large wok or 12-inch skillet over medium-high heat until the mustard seeds start to sizzle and pop, 1 to 2 minutes (use a splatter screen, if you have one, so the seeds don't pop out of the pan). Add the curry leaves (if using), chile, coriander, cumin seeds, and turmeric and cook, stirring occasionally, until the cumin browns and the curry leaves are crisp, 1 to 1-1/2 minutes. Stir in the garlic and jalapeno and cook until the garlic is fragrant, about 30 seconds.

Drain the potatoes and add them to the pan along with the onions. Cook, stirring occasionally, until the potatoes are translucent around the edges, 2 to 3 minutes. Cover, reduce the heat to medium-low, and cook, stirring and scraping the bottom of the pan every 5 minutes, until the potatoes are just tender, 12 to 15 minutes. (Reduce the heat to low if the potatoes seem to be burning.)

Add the salt and cayenne (if using) and cook for 30 seconds. Stir in the mint, cilantro, and lemon juice, cover the pan, and let the potatoes sit off the heat for 10 minutes. Scrape up the browned bits and stir them into the potatoes. Taste, add more salt if needed, and serve.