Meanwhile, trim the leeks, and finely chop. Wash the chopped leeks thoroughly to get rid of any grit. Place a medium pan over a low heat, melt the butter and very gently sweat the leeks with the garlic, 4 large pinches of flaky sea salt and a good pinch of black pepper. After about 10 minutes the leeks will have a translucent, glossy appearance. Take off the heat and leave to cool. Once the leeks have cooled, drain off any lingering fluid and add them to the soaking pasta.