In a saucepan over medium-high heat, bring milk and bay leaf nearly to a boil. Meanwhile, in another saucepan over medium heat, melt 1/4 cup butter. Add flour to butter and cook, whisking constantly, for 2 minutes. Remove bay leaf from milk; slowly add milk to butter mixture, whisking constantly. Stir in mustard, nutmeg, salt and pepper. Cook, stirring occasionally, until thickened, about 5 minutes. Remove from heat; add Cheddar cheese and stir until cheese is melted. Fold in green beans and mushrooms; transfer to a buttered 2-quart casserole dish. Melt remaining butter; toss with bread crumbs. Sprinkle Parmesan cheese and bread crumbs over casserole. Bake, uncovered, at 350 degrees for 50 minutes, or until bubbly and golden.