In a small bowl, whisk together the eggs, milk, pepper and salt.
Place butter into a large skillet and heat over medium heat, then pour the whisked eggs into the melted butter.
Place two pieces of bread on top of the egg mixture right next to each other. Flip the bread over so that both sides get wet, then allow the eggs to set.
Once set, slide a spatula under the middle of the eggs and flip the bread and eggs over.
Put the sautéed onion, bacon, and cheese on top of the bread.
Fold the overlapping egg edges onto the bread. Flip one bread over onto the other, creating a sandwich.
Remove the sandwich from the skillet, cut it in half and serve it hot.
Notes
Be sure to precook any ingredients like meats or sauté any vegetables like onions or mushrooms!