Gently push out the inner leaves of each piece of leek leaving six outer shells (each with two layers of leek) that resemble cannelloni tubes.
Bring a pan of water to the boil, add the leek ‘tubes’ and boil for 1-2 minutes until just tender. Drain and refresh under cold running water. Drain on kitchen paper and set aside.
Finely chop all the inner leaves. Heat the oil in a frying pan and fry the chopped leeks and onion over a medium heat for 4-5 minutes, stirring frequently. Add the mushrooms and fry over a high heat for a further 5 minutes until the leeks are soft. Cool then stir in the chopped herbs and ricotta. Preheat the oven to 200ºC / fan 180ºC / gas mark 6.
Spoon the mixture into each leek tube and place, side by side, in a small ovenproof dish. Scatter over the breadcrumbs and cheese. Bake in the preheated oven for 15-20 minutes until golden.