In a bowl, with a whisk, whisk eggs, milk and Dijon mustard. Add croutons, ham and half the cheese, half bell peppers and half green onions. Pour the mixture of eggs in a greased pie plate of 9 inches or an 8 inches greased square pan. Sprinkle with remaining cheese, bell pepper and green onion.
Cook in a preheated oven of 350 F for about 20 minutes or until the frittata is golden and the eggs are set. Keep cooking under the preheated grill for about 2 minutes or until frittata is golden and is lightly puffed. Put the pan on a rack and let cool for 5 minutes. Use a spatula to loosen the edges from the pan and cut in wedges.