Fill a large bowl with ice water. Bring a large saucepan of water to a boil. Add 1⁄2 teaspoon kosher salt per cup of water. Add the beans and cook until just tender, as few as 5 minutes for fillet beans and up to 25 minutes for larger beans. Use a slotted spoon or strainer to immediately transfer into the ice water to stop the cooking and set the color.
Drain the beans and spread them in a shallow layer over a few sheets of white paper towel or a clean tea towel and roll up loosely into a log shape. Put the roll in a zip-top bag and refrigerate for up to 4 days.