Don’t worry about the insane amount of olive oil called for in this recipe—most of it gets drained off after you cook the potatoes. Feel free to strain it and save it in the fridge to use the next time you cook potatoes.
Trim the leeks or spring onions. Cut them in half lengthwise and thinly slice them—including the darker green tops. If you encounter any dirt or grit in the process, clean them: put the slices in a large bowl, cover them with cool water, swish around to clean, and lift the slices out of the water (don’t dump the dirty water back on them!).
In a large pot over medium heat, add 2 tablespoons of the oil. Add the leeks or onions and the salt. Cook, stirring now and again, until the leeks or onions, including their green tops, are soft, about 10 minutes.
Meanwhile, peel and thinly slice the potatoes. When the leeks or onions are soft, add the potatoes and the remaining oil to the pot. Cook, stirring occasionally and making sure to scrape up any bits sticking to the bottom of the pan, until the potatoes are tender—they won’t really cook more once in the egg mixture, so make sure they’re soft!
Meanwhile, peel and mince the garlic. Mince the herbs.
When the potatoes are just soft, add the garlic and cook, stirring frequently, until fragrant, about 1 minute. Remove from the heat and stir in the herbs. Set a colander or strainer over a large bowl and drain the potato mixture.
While the potatoes drain, whisk the eggs together in a large bowl. Add the potatoes mixture and stir to combine everything.
Heat a 12-inch frying pan over medium heat. Once it’s hot, spray liberally with spray oil (or use a nonstick frying pan). Add the potato-egg mixture, using a spatula to arrange it evenly in the pan. Reduce the heat to low, cover, and cook until the eggs are set, about 20 minutes. The tortilla should not jiggle in the middle when you shake the pan. The result is more important than time here, so watch the tortilla over the clock.
If you want, you can pass the tortilla under a broiler to brown the top, which will become the bottom.
Turn out onto a large plate or serving platter. Serve—cut into slices—hot, warm, or at room temperature.
Do as the Spanish do and slice up any leftovers and tuck them into a sandwich.