Boil the Eggs: Place the eggs in a saucepan, add enough water to cover them, and bring to a boil over medium heat. Once boiling, reduce the heat. Boil the soft-boiled eggs for 7 minutes and the hard-boiled eggs for 10 minutes. Transfer to cold water and peel.
Make the Egg Salad: Mash the hard-boiled eggs in a bowl with a fork or masher. Add mayonnaise, yogurt, vinegar, salt, and pepper, then mix well.
Assemble the Sandwich: Spread mustard on the bread slices. Add a layer of egg salad, place a soft-boiled egg in the center, and cover it with more egg salad. Place another slice of bread on top and gently press down.
Serve: Slice the sandwich in half, garnish with parsley, and enjoy!
Notes
Adjust the mayo to your liking—add more for extra creaminess or use less for a lighter feel.
When putting the sandwich together, gently press the bread so the egg salad stays in place when you cut it. You can also wrap it up and let it sit for 5 minutes before slicing—it helps everything settle.
If you want that classic Japanese convenience store look, go ahead and trim the edges!