Method

1. Heat the oven to 180°C/160°C fan/gas 4. For the white sauce, heat the milk in a heavy-based pan with the bay leaves, sliced onion, celery and peppercorns. When the mixture reaches a simmer, remove the pan from the heat and leave to infuse.
2. Bring a large pan of salted water to the boil and add the kale and the leeks. Cook for 3-4 minutes or until the leeks are just tender. Drain well, then set aside.
3. Heat a knob of butter and 1 tbsp olive oil in a large frying pan over
   a high heat. Add half the mushrooms and season with salt and pepper. Fry, stirring often, for 5-10 minutes, making sure the liquid released by the mushrooms has evaporated and they have a bit of colour. Stir in half the garlic and half the thyme, cook for a minute more, then transfer to a plate. Repeat for the remaining mushrooms, thyme and garlic. 
4. Heat the butter for the white sauce in a medium heavy-based pan. Scatter over the flour and, using a wooden spoon, work it into the butter. Cook gently, stirring, for 1 minute. Set a sieve over the pan and pour in half the infused milk (discard the flavourings). Work this into the butter and flour mixture. As it thickens, pour in the remaining milk, then switch to a whisk and whisk the sauce until thick and smooth. Add half the blue cheese, the mustard and salt and pepper to taste.
5. Mix a third of the white sauce into the kale and leeks, then taste and season with salt and pepper. Arrange one third of the kale and leeks over the base of the baking dish. Lay over a third of the pasta. Arrange another third of the greens over the pasta, followed by half the fried mushrooms. Pour over half
   the remaining white sauce and a another third of the lasagne sheets.
6. Spoon over the last of the greens, then top with the rest of the mushrooms. Put the remaining lasagne sheets on top, then pour over the rest of the white sauce and smooth the top with the back of a spoon. Combine the remaining cheese and the breadcrumbs, then scatter this over the top of the lasagne. Drizzle with a little olive oil, then bake for 40 minutes.