Make the omelet: Crack the eggs into a bowl, add milk, salt, and pepper, then whisk well. Heat oil in a pan over medium heat. Pour in the egg mixture and cook gently, folding and shaping it to fit the size of your bread as it cooks.
Assemble the sandwich: Spread mayonnaise and mustard on the bread. Place the omelet on one slice, then top with another slice of bread.
Serve: Slice in half and enjoy!
Notes
Keep the center slightly soft. It will continue to cook after you take it off the heat.
Wrap the sandwich in plastic wrap before cutting, then slice through the wrap to help it hold its shape.
You can lightly toast the bread if you like. It adds a nice texture.