Preheat oven to 375 degrees.
Prepare a 9×13 baking sheet with non-stick cooking spray, a layer of parchment paper, and another spray for non-stick cooking spray. Also fold the corners to keep egg mixture from running under the parchment paper.
To a blender, add eggs, milk, flour, salt, pepper, and mustard. Process until smooth and frothy. Pour onto prepared baking sheet.
Bake at 375° until eggs are set, 20-25 minutes or until eggs are set.
While the eggs bake, heat a skillet over medium-high heat. Melt the butter in the skillet and add the onions and bell peppers. Saute until softened. Stir in ham and heat through.
Remove the eggs from oven. Sprinkle with cheese. Add a layer of the vegetables and ham.
Roll up the omelet, starting with a short side, using the parchment paper to create an even roll. Place with the seam side down on a serving platter.
Sprinkle with additional cheese and vegetables if desired.
Let the omelet roll rest for 2 to 3 minutes to allow the cheese to melt. Cut into slices and serve.