Preheat the oven to 400 degrees F.
Trim the top leaves and stalks off the cardoons until they resemble a celery-sized plant about 14 to 16 inches in length. Remove each stalk and peel off the outer fibrous ribs and hard green skin, using a paring knife.
Fill a stock or pasta pot 2/3 to 3/4 full (about 10 cups) with water and add lemons, which have been halved and squeezed into the water. Cut each cardoon into 3 pieces, approximately 5 to 6 inches in length, placing immediately into the acidulated water to avoid discoloration. Place on medium-high heat and bring to a boil. Lower the heat to medium and simmer for 1 hour. Remove, drain and rinse. Allow to cool.
Lay the cardoon pieces in an oven-proof casserole and cover with 2 cups besciamela sauce. Sprinkle with the grated cheese and bread crumbs and bake for 30 to 35 minutes, until dark golden brown and very bubbly. Serve immediately.
In a medium saucepan, heat butter until melted. Add flour and stir until smooth. Over medium heat, cook until light golden brown, about 6 to 7 minutes.
Meanwhile, heat milk in separate pan until just about to boil. Add milk to butter mixture 1 cup at a time, whisking continuously until very smooth and bring to a boil. Cook 10 minutes and remove from heat. Season with salt and nutmeg and set aside.
Recipe copyright 2000, Mario Batali. All Rights Reserved.