After soaking the beans for at least 12 hours, strain them and put them to boil in a saucepan full of cold water over a low heat, for roughly an hour (taste them to make sure), with 2 sage leaves, 1 spoonful of olive oil and a pinch of salt (this latter added at the end of cooking). Five minutes before they have finished cooking, chop the spring onion, put it in a frying pan with the oil and sauté over a moderate heat. Strain the boiled beans and tip them into the frying pan, season with salt, pepper and the fennel. Stir everything together slowly, over a low heat, for approximately ten minutes. Pour the beans (which due to the fact that they're cooked twice – in the saucepan and the frying pan – will now be soft) into two small well-greased moulds, approximately 8 cm wide and 10cm tall, pressing them down well. Put the moulds in the oven at 180 °C for 10 minutes. Straight afterwards turn the oven up to 250 °C, lay the slices of Pancetta in a baking pan, put them in the oven and cook for about 5 minutes, until crispy. Turn out the flans while still hot and serve them decorated with the Pancetta.