Heat oven to 350 degrees. Coat 9x13 baking dish with cooking spray.Toss onion, bell peppers, broccoli and 1 cup cheese into cooking dish, spreading evenly.In a medium mixing bowl, whisk eggs, ground mustard, half and half and whole wheat flour. Pour mixture over vegetables and cheese.Bake uncovered for 45 minutes. Remove and top with remaining 1 cup cheese. Return to oven for 10 minutes.Allow to rest for 5-10 minutes before serving. Enjoy your Cheesy Vegetable Breakfast Casserole!