Preheat oven to 400 degrees.
Trim top leaves and stalks off cardoons until it resembles a celery-sized plant about 14 to 16 inches in length. Remove each stalk and peel off outer fibrous ribs and hard green skin using a paring knife.
Fill a stock or pasta pot two-thirds or three-quarters full (about 10 cups) with water and add flour and lemons, which have been halved and squeezed into water. Cut each cartoon into 3 pieces approximately 5 to 6 inches in length and continue until all cartoon is cleaned, trimmed and cut. Place on medium high heat and bring to a boil. Lower heat to medium and cook just below boiling for 1 hour. Remove, drain, rinse and allow to cool. Lay cartoon pieces in oven-proof casserole and cover with 2 cups besciamela sauce. Sprinkle with grated cheese and bread crumbs and bake for 30 to 35 minutes until dark golden brown and very bubbly. Serve immediately as an appetizer or light lunch with grilled bread and crushed red pepper flakes.