This soup is from Provence, where many hearty soups that feature potatoes, saffron, and a poached egg are referred to as bouillabaisse. This recipe is from the vegetarian cookbook Mediterranean Harvest.
Bring the water to a boil in a large pot. Add 1 T salt and the spinach. Blanch just until wilted, less than a minute. Using a slotted spoon, transfer the spinach to a bowl of cold water. Let sit for a couple of minutes, drain, and squeeze out the water. Chop coarsely and set aside. Measure out 6 c of the cooking water and set aside.
Heat 1 T olive oil in a large, heavy soup pot over medium heat and add the leeks. Cook, stirring often, until tender and translucent, about 5 minutes, and add the garlic. Cook, stirring, for another minute or so, until fragrant.
Add the spinach and stir together for a few seconds, then add the potatoes, the 6 c reserved blanching water, and the bouquet garni. Bring to a boil and crumble in the saffron.
Reduce the heat to a simmer, cover, and simmer for 15 minutes, or until the potatoes are tender. Taste and adjust salt. Add pepper. Discard the bouquet garni.
Have the soup at a bare simmer. One by one, break each egg into a teacup, then tip into the soup. Poach the eggs for 4-5 minutes, until just set.
Place a crouton in each bowl and, using a slotted spoon, scoop out a poached egg and place it on top. Ladle the soup into the bowls. Sprinkle on the cheese and drizzle on a little olive oil. Serve.