Preheat your oven to 350°F (175°C). Line the large baking sheet with parchment paper or spray it with non-stick spray.
In a large mixing bowl, whisk together the eggs, milk, salt, and black pepper until well combined.
Add the diced bell peppers, onions, and cooked sausage or bacon (if using) to the egg mixture and stir gently to distribute evenly.
Pour the egg mixture onto the prepared baking sheet, spreading it out evenly.
Sprinkle the shredded cheese over the top of the egg mixture.
Bake in the preheated oven for 20-25 minutes, or until the eggs are set and slightly golden on top.
Once cooked, remove the baking sheet from the oven and let it cool for a few minutes. Then, cut the egg slab into squares that will fit your sandwich rolls or English muffins.
Assemble the breakfast sandwiches by placing a square of the baked egg onto each sandwich roll or muffin. Add any additional toppings or condiments, if desired.
Serve immediately, or wrap and refrigerate for easy grab-and-go breakfasts throughout the week.