At the market, the cardoon looks somewhat like mutant celery, with jagged spikes that run along each of its fibrous stalks. It's only when you see the plant growing in the field that you notice its greater resemblance to the artichoke. It's the stalks of the cardoon plant that are commonly eaten as a vegetable; with the artichoke plant, it's the immature flower.
The two plants are closely related and have a similar, sweet flavour (although you'll occasionally find bitter stalks, if they have been left too long in the soil).
Preparing the cardoon involves a lot of work. You need to trim off and discard the spikes from each stalk, and then cut off the toughest exterior fibres. As with artichokes, the vegetable discolours once you cut into it, so the pieces need to be soaked in acidulated water (water with vinegar or fresh lemon juice). After cutting up the cardoon, simmer it in salted water or light chicken stock to soften the fibres until tender enough to eat.
The cardoon's fibrousness can be off-putting. I like cardoon soup, where the vegetable is puréed and strained so it's silky smooth rather than stringy. Heat equal amounts of unsalted butter and oil in a soup pot and add chopped carrot, onion and celery, and season with salt. Cook until the vegetables are tender, then add chopped cardoon stalks. Add light chicken stock to cover the vegetables and bring to the boil. Lower the heat and simmer until the cardoon is tender. Purée the mixture in a food processor then put the purée through a food mill.
If you want really smooth, delicate soup, strain the mixture through a fine sieve. Put the purée back into the rinsed-out pan and heat it until it simmers. Stir in some heavy cream and if the mixture is too thick, add more stock. Season to taste then serve with croutons that have been fried in clarified butter until crisp.