Cut the avocados in half, remove the pit, and scoop the flesh into a bowl.

Add lemon juice, salt, and black pepper to the bowl and mash until smooth.

Toast the bread slices until golden brown.

Spread a generous amount of the avocado mixture on four slices of toasted bread.

Layer with spinach, tomato, cucumber, and cheese if using.

Top with the remaining slices of bread and cut in half to serve.