Preheat the oven to 350°F. Spray a half sheet pan with cooking spray, fit it with a sheet of parchment paper pressing it onto the sprayed pan to encourage it to stick to the pan. Then spray the sheet of parchment paper.
Beat the eggs. Stir in the milk, flour, garlic, salt and pepper. Beat until combined with no lumps of flour. An immersion blender makes quick work of this if you have one.
Pour egg mixture onto the pan. It’s ok if it doesn’t go all the way to the edges. Sprinkle with ham, bell pepper and onion.
Bake 15 minutes or until the edges are set. Sprinkle with most of the cheese, leaving a little for garnish.
Bake 2 more minutes or until the cheese is melted.
Roll up the omelet like you would a swiss roll, using the parchment paper to help you roll the eggs. Peel off the parchment as you go.
Place on platter, sprinkle with remaining cheese and diced tomato for garnish. Cut into slices and serve.