01 - In a mixing bowl, vigorously whisk the eggs, milk, salt, and black pepper until combined.
02 - Melt 1 tablespoon butter in a large nonstick skillet over medium heat.
03 - Pour half of the egg mixture into the skillet and tilt to ensure even coverage.
04 - Quickly dip two slices of sandwich bread into the mixture in the skillet, flip to coat both sides, and arrange them side by side in the skillet.
05 - Allow the eggs to cook undisturbed for 1 to 2 minutes until just set.
06 - Using a spatula, gently flip the entire egg-and-bread combination so the bread faces down on the skillet.
07 - Layer 2 slices cheddar cheese, half the bacon, tomato, avocado, and spinach on one slice of bread.
08 - Fold any overhanging cooked eggs over the bread slices, then fold one piece of bread over the other, enclosing the fillings.
09 - Press the sandwich lightly and cook for 1 to 2 minutes per side, until golden brown and the cheese is fully melted.
10 - Repeat steps with remaining ingredients to make the second sandwich.
11 - Slice each sandwich and serve warm, optionally accompanied by mayonnaise or sriracha mayo.