Scrape each artichoke leaf and cut the hearts into large pieces. Set aside. In a preheated dry frying pan (on a low flame), stir the flour until lightly browned. Set aside. Melt the butter (in a skillet). Add the onions and saut about 5 minutes. Add the flour to the butter mixture. Stir until smooth. Mix in the remaining ingredients, except the artichokes and breadcrumbs. Simmer for 10 minutes. Place the artichokes in a casserole. Add the oyster mixture and top with the breadcrumbs. Dot with butter and bake in a 350-degree oven for 15 to 20 minutes.
Makes 6 to 8 servings.