Preheat oven to 220°C (430°F). If using a fan oven then preheat to 200°C (390°F).
Get a large mixing bowl, and crack 12 eggs. Be careful not to get any egg peel in the bowl.
Whisk the eggs until blended, whisk in milk, and season with salt and pepper.
Lightly grease a sheet pan with non-stick oil or brush over some vegetable oil.
Pour the egg mixture in the sheet pan, and add your toppings (mushroom, red bell pepper, tomato, onion, green onion, and cheddar cheese)
Pop in the oven on the lower rack, and bake for 15-20 minutes. The baking time may vary depending on what size of sheet pan you're using, and how many toppings you've added but the center should set and no longer be jiggly which means that the eggs are done.
Cut into 12 squares, and serve.
Use very fresh eggs, the fresher the better. If you can afford it, go for organic eggs they taste much better.
Make sure that you're using a rimmed sheet pan. The egg mixture is basically liquid, and it won't work if you use a flat cookie sheet pan.
Use a whisk to whisk the eggs rather than a mixer or a blender. Whisk until everything is foamy, but do not over whisk.
Do not overcrowd the omelet with too many toppings. It's a different form of omelet or a frittata overall! Plus, the vegetables release lots of liquid and too much liquid can turn the omelet into a mess as the eggs won't set easily.