Heat the milk gently with the onion, bay leaves and peppercorns - stirring - until it begins to simmer, then turn off, cover and set aside for 20 minutes or so.
In the meantime, wash the spinach and cavolo nero and place in a saucepan. Put a lid on and gently heat until wilted. Drain, then run under cold water until cool. Place in a clean tea towel and squeeze out as much water as possible. Chop finely.
Heat a knob of butter in a pan and gently cook the sprouts and garlic until softened and smelling great. Combine with the spinach and cavolo nero. Add the ricotta and mix well. Season with salt and set aside.
Cook the pasta shells in boiling salted water until just half cooked (they will continue cooking in the oven later). Run under cold water to cool them down. Stuff each with some of the greens and ricotta mixture. Set aside in a bowl.
To finish the bechamel, strain the milk and add back to the pan. Add the flour and butter and heat on a medium heat, whisking all the time until nice and thick. Melt in the cheese and taste - add salt if necessary. Cover and set aside.
Preheat the oven to 180°C.
Cover the base of a large ovenproof dish with the bechamel. Arrange the pasta shapes on top, stuffing side up. Crumble the remaining blue cheese on top and cover with grated Parmesan. Bake for 20-25 minutes until bubbling and beginning to golden. Finish under the grill for 5 minutes to brown the top (watch carefully!).