Place eggs in a pot and cover with water. Bring to a boil and cook for 10 minutes until firm.

Transfer eggs to cold water and let them cool completely before peeling.

Peel the eggs and place them in a mixing bowl. Mash gently with a fork, keeping small chunks for texture.

Add mayonnaise, mustard, chopped onion, and chopped cucumber.

Season with salt and black pepper. Mix until creamy and well combined.

Lay out bread slices and spread butter evenly on one side of each slice.

Spoon the egg mixture onto one slice and spread evenly to the edges.

Place another slice of bread on top and press gently.

Cut into halves or triangles and serve fresh.