Blanch cardone in lightly salted boiling water for 3 minutes. Drain and cool.
Heat olive oil and butter in fry pan and saute onion until tender.
Coat cardone with flour and add to sauteed onions, braise for a few minutes and then add broth, salt and pepper.
Simmer for 30 minutes, remove from heat and puree soup with hand blender. At the bottom of each soup bowl, place a slice of toasted bread and pour boiling soup over bread to soften. Garnish with bacon if desired.