1. Preheat the oven to 350°F.
2. In a bowl, mix together ¼ cup of butter and breadcrumbs, set aside.
3. In a saucepan, heat milk and bay leaf, on medium high heat.
4. In another saucepan, over medium high heat melt ¼ cup of butter.
5. Add flour to butter and cook whisking continuously for 2 minutes.
6. Take the bay leaf out of milk, and discard.
7. Stream the milk slowly into the butter, whisking constantly to blend well.
8. Add in Dijon mustard, nutmeg, salt and pepper.
9. Cook the milk mixture, for 5 minutes, stirring occasionally until thickened.
10. Take the saucepan, off the heat and add cheese.
11. Stir the milk mixture until the cheese melts.
12. Add mushroom and green beans into the milk mixture and fold together.
13. In a greased 2 quart casserole dish, put the milk vegetable mixture.
14. Sprinkle Parmesan cheese and breadcrumbs mixture over the casserole.
15. Bake uncovered for 50 minutes or until the top gets golden colour.
16. Remove from the oven and allow to rest for 5 minutes.
17. Serve Green Bean & Wild Mushroom Casserole hot.
You can substitute mushrooms with any other vegetable of your choice.