Slice bacon. Peel potatoes and cut them into half inch cubes. Chop onion.
In a saucepan, heat butter, add bacon, onion and potatoes. Sauté for a couple of minutes. Add water and cook over medium heat until potatoes s are tender removing the scum periodically. Then add spinach and soymilk.
Turn off the heat just before it starts to boil, add miso and whisk to dissolve miso paste with chopsticks or fork until smooth.
Pour the soup into each serving bowl and sprinkle black pepper if you desire.