Mushroom Lasagna - Deliciously Yorkshire

Ingredients

• 1 small carrot
• 1/2 small onion
• 1/2 stick celery
• 1 fresh bay leaf or 2 dried
• 500ml milk (plus a little extra if needed)
• 55 g plain flour
• 55 g butter + 25g for baking
• Sea salt and freshly ground pepper
• 125g Parmesan (sliced very thinly) + 25g grated
• 450g mixed mushrooms, thickly sliced