Mix the flour, yeast and salt together in a mixing bowl. Whisk in enough water to obtain a smooth, pourable batter. Cover the bowl with a towel and let rest in a warm place for at least two hours.
Using a paring knife, trim the cardoons stalks of their leaves and prickly bits along the edges, as well their filaments and any discolored bits.
After you finish trimming a stalk, cut it into whatever lengths you like and immediately place the pieces into a large bowl of water which you will have acidulated with a freshly squeezed lemon.
Boil the cardoons in well salted water (along with a spoonful of flour) until the pieces are tender with just a trace of "bite"to them and have lost their bitterness, usually about 15-20 minutes.
When you're ready to eat, heat enough oil in a skillet to come up at least 3 cm (1 inch) from the bottom. The oil should be very hot but not smoking.
Dip the cardoon pieces into the batter, let the excess run off and fry them in the hot oil until they are golden brown on both sides. As they are done, transfer the fried cardoon pieces to a baking rack or platter lined with paper towels.
When you're done, sprinkle the fried cardoons with salt and serve right away, with lemon slices or wedges if you like.