Trim the ends and dark green tops from the leeks and discard. Cut the leeks in half lengthwise, then crosswise into 1-inch pieces. Immerse them in a bowl of cold water, swish to remove any grit and lift them out into a colander. Clean the bowl, change the water and repeat until the water remains clear. You should have 6 cups leeks. Once they are all in the colander, toss them a bit with your fingers to separate the leaves.
Heat a large skillet over high heat until hot. Add 1 tablespoon of the oil and swirl to coat the bottom of the pan. Add half the leeks and a generous pinch each salt and pepper. Spread the leeks into a single layer and cook, without stirring, until slightly charred, about 1 minute. Stir in a small pinch pepper flakes and continue cooking, without stirring, 1 minute. Toss and stir until the leeks just start to wilt, about 1 more minute. Transfer to a bowl. Repeat with 1 tablespoon of the oil, the remaining leeks, a generous pinch each salt and pepper and another small pinch pepper flakes, adding the leeks to the bowl when done.
Heat the remaining tablespoon of oil in the skillet (let it get really hot). Add the shrimp, garlic and another pinch pepper flakes (or to taste). Season with salt and pepper, and cook, tossing, until the shrimp just starts to turn pink in spots, less than 1 minute. Add the leeks and toss to combine; the shrimp should now be pink. Serve immediately over rice if using.
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