Start by preheating your oven to 375°F (190°C). Grab a standard 13×18 inch half-sheet pan. This is the most important step for success: line the pan with parchment paper and then lightly spray the paper and the sides of the pan with non-stick cooking spray. This double-layer of protection ensures your eggs will slide right out without sticking or tearing.

In a large mixing bowl, crack all 15 eggs. Pour in the milk, salt, black pepper, and garlic powder. Use a large whisk or an immersion blender to beat the eggs until they are completely combined and slightly frothy. Whisking well incorporates air, which results in a fluffier omelet once baked.

Spread the diced ham, green peppers, red peppers, and onions evenly across the prepared sheet pan. Sprinkle half of the shredded cheddar cheese over the toppings. Pour the egg mixture carefully over the ingredients, ensuring the liquid reaches all four corners of the pan. Use a fork to gently nudge the ham and vegetables so they are distributed evenly throughout the egg.

Place the sheet pan on the middle rack of the oven. Bake for 20 to 25 minutes. You will know it is done when the center is set and no longer jiggles when you move the pan. The edges should be slightly golden brown. During the last 2 minutes of baking, sprinkle the remaining cup of cheddar cheese over the top and let it melt into a gooey layer.

Remove the pan from the oven and let it rest for 5 minutes. This cooling period allows the eggs to firm up slightly, making them much easier to cut. Slice the omelet into 12 even squares using a sharp knife or a pizza cutter. Garnish with fresh chives or parsley and serve warm with a side of toast or fresh fruit.