1. Preheat the oven to 180C. Melt the butter in a large saucepan and stir in the bay leaf, mushrooms and leeks. Simmer for 5-10 minutes until soft.
2. Add the cooked pork and heat through thoroughly. Add half the milk and simmer for a few minutes.
3. Sieve over the flour, stirring all the time to avoid lumps, and allow to thicken, slowly adding the rest of the milk to make a sauce. Remove the bay leaf. Arrange on the bottom of a oven-proof serving dish.
4. In a large bowl add the butter, flour, mustard and herbs and knead together. Add the cheese and oats and knead further.
5. Distribute the crumble evenly over the top of the base mixture.
6. Cook in the centre of the oven for 30 minutes until the crumble is crunchy and well browned.