Cut the avocado in half, remove the pit, and peel it.
Slice the avocado.
Toast Shilla Myunggwa bread until golden brown.
Spread a thin layer of mustard or mayonnaise on the bread.
Place the sliced avocado on top.
Season with salt and pepper.
Optionally, add cherry tomatoes, salad greens, or cheese to your taste.
Cover with another slice of bread to finish the sandwich.