Rinse the clams then place them in a large bowl of ice-cold water and mix in the flour. Leave at room temperature for 30 minutes. Lift the clams up with your hands and wash again. Any clams that remain open after being tapped on a bench should be discarded. Store in the fridge until needed.
Heat the oil in a large deep frying pan with a lid over medium heat. Add the onion and fennel and