Preheat the oven to 180°c.
In a large pan fry the shallots, carrot and celery in the butter, until soft. Add 800g of the mushrooms, the garlic, oregano and thyme and cook until golden. Put to one side.
Start the bechamel by heating the butter in a saucepan until melted and add the shallots and garlic. Cook until soft and then add the flour. Cook stirring rapidly for 3-4 minutes.
Add the milk and whisk until smooth and thickened. Add the mustard, then mix in the Parmesan and season well with salt and black pepper.
Assemble the lasagne by spreading half of the mushrooms in the base of the dish, then a third of the cheesy bechamel. Top with a layer of pasta sheets and a third of the mozzarella. Repeat these layers, and finish with a final layer of the bechamel, mozzarella and Parmesan.
Bake for 20-25 minutes until the top is golden brown.
While the lasagne is cooking, spread the remaining mushrooms out on a tray and drizzle with oil and salt. Put into the oven for the last 15 minutes of cooking time until golden and crisp.
Remove the lasagne the oven, top with the crispy mushrooms and rest for 10 minutes before serving.
