Kale and mushroom lasagne

Name: Kale and mushroom lasagne
Prep time: 30 min
Cook time: 45 min
Total time: 1 h 15 min
Cuisine: European, Italian, Mediterranean
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 and freshly ground black pepper; For the béchamel sauce; 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