Make sauce: In a saucepan scald milk and bay leaf. In another saucepan, melt butter over medium heat. Add flour and cook, whisking constantly, 2 minutes.
Remove bay leaf from milk and slowly pour into butter-flour mixture, whisking constantly. Season, to taste, with mustard, nutmeg, salt and pepper. Cook, stirring occasionally, until thickened, about 5 minutes. Remove from heat and add cheddar, stirring until melted and smooth.
Heat oven to 350 degrees F. Fold green beans and mushrooms into sauce, then transfer to a buttered casserole. Sprinkle top with Parmesan and buttered bread crumbs. Bake 50 minutes, or until casserole is bubbly and topping is lightly browned.