Prep Work: Cut onion; store separately. Chop bell pepper and mushrooms store together.
Heat 2 Tbsp. olive oil in large oven- safe skillet and cook onions 4 min., or until just softened.
Add bell pepper and mushrooms; cook 4 min. more and add green beans. Stir to combine.
Cook 8 more min., or until vegetables softened and most liquid evaporated. Pour off extra liquid after 9 min. if still there. Stir in half of the salt and pepper.
While vegetables are cooking, melt 4 Tbsp. butter in another (smaller) skillet. Then whisk in flour for 2 min., or until it begins to turn golden colored. Add milk, cayenne, nutmeg, the other half of salt and pepper and whisk until it thickens, approx. 3 min. Should be like gravy on the thick side. Preheat broiler.
Remove from heat and cool for a few minutes; then combine gravy with vegetables. Gently mix in 2/3 of the parmesan into the vegetables, and then sprinkle the remaining parmesan and all of the bread crumbs over top. Cover with foil.
Broil with foil on for 10 min. at least 8 inches from the heat. Take foil off and brown the top for approx. 30 seconds. Sprinkle with some Chinese noodles for that extra crunch if you want it.
(adapted from marthastewart.com "Green Bean Casserole")