Prepare all ingredients: bread, eggs, onion, mustard sauce, mayonnaise, salt, and pepper.

Boil the eggs until fully cooked.

Cool the boiled eggs in cold water.

Chop the onion finely.

Mash the boiled eggs using a fork.

Mix the mashed eggs, chopped onion, mayonnaise, mustard sauce, salt, and pepper together.

Wrap the bread slices in plastic wrap and microwave for 1–2 minutes.

Place the egg mixture in the center of the bread slices.

Use a small bowl to press and shape the sandwich.