To get the cardoons ready, start by pulling the bunch apart. Slice off the narrow ends and any leaves attached. Use a vegetable peeler to remove the tough strings (they're like celery strings) off the outer side of the cardoons. You can also just peel them off with your fingers. When you get to the inner stalks, rub off the white "bloom" from the stalks; this is bitter. Chop the cardoons into 3-inch pieces, putting each piece into a bowl of water that has the juice of a lemon squeezed into it.
Bring a large pot of water to a boil. Add the salt and the juice of another lemon, then whisk in the flour; this process helps keep the cardoons that pretty light green. Boil until they are tender, which can take anywhere from 20 to 45 minutes. The inner pieces will cook faster. I fish the pieces of the inner stalk out and set them aside.
When the cardoons are done, take about 2 cups' worth of cardoon stalks -- from the outer stalks -- and puree them in a blender with 2 cups of water. Mix this puree with another two cups of water and pour into a pot. Bring to a bare simmer and have this at the ready. Chop the remaining cardoon stalks small; they will go whole into the risotto.
In a saucepan heat 3 tablespoons of butter over medium-high heat. Add the white parts of the green onion and saute for 1 minute. Add the garlic and the rice and saute another minute, making sure to coat the rice well. Turn the heat to high and add the white wine. Stir it constantly until the wine has pretty much evaporated.
Add two ladles' worth of the cardoon broth to the risotto. Cook over medium-high heat, stirring more or less constantly, until it has mostly boiled away. Repeat this process until the rice is tender, but still a bit al dente. If you run out of cardoon broth (which should only happen with old rice), use water.
When the rice is ready, add back the chopped cardoon pieces, the green parts of the green onion, the remaining tablespoon of butter and the parmesan cheese. Stir well and loosen the risotto up with a little water if need be -- I like my risotto all'onda, or a little porridge-like. Grind some black pepper over everything and add the truffle oil, if using. Serve with the same white wine you put in the risotto.