You have to pick the part of the plant just below the waterline, you can just twist and rip it off or use a knife for a clean cut.
Then you remove all the green and cut the white part into small slices (like you’d cut an onion). Then fry the slices in a pan with butter, when the pieces become translucent they’re done. Add a little dash of pepper, salt and few drops of lemon juice.
The taste is a bit “almondy”, it’s great with fried halloumi or beef.
The best time to pick the roots is before Midsummer, after that they’re not as tasty.