Kale and Mushroom Lasagne

Ingredients

• About 300g curly kale or cavolo nero, tough stalks removed
• 30g butter
• 500g mushrooms, sliced
• 2 garlic cloves, finely chopped
• A few sprigs of thyme, leaves only, chopped
• 175g lasagne sheets (fresh is best, but dried is fine)
• 20g Parmesan, hard goat’s cheese or other well-flavoured hard cheese, grated
• A little rapeseed or olive oil
• Sea salt
• freshly ground black pepper
• 750ml whole milk
• 1 bay leaf
• 1 onion, roughly chopped
• 1 celery stalk, roughly chopped
• A few black peppercorns
• 50g unsalted butter
• 50g plain flour
• 2 teaspoons Dijon mustard