Rosemary has a strong flavor, but offers only a subtle hint in this spinach soup. If you like, any seasonal greens you have on hand can be substituted for the spinach.
To prepare croutons: Preheat oven to 375°F.
Toss bread cubes, oil, garlic and rosemary in a large bowl until well combined. Spread in a single layer on a large baking sheet. Bake until golden and crisp, 12 to 15 minutes.
Meanwhile, to prepare soup: Melt butter in a large saucepan over medium heat. Add onion, garlic, rosemary, salt and pepper, reduce heat to medium-low and cook, stirring occasionally, for 5 minutes. Stir in potatoes and cook, stirring occasionally, for 3 minutes. Pour in broth (or water). Bring to a simmer over medium heat and cook until the potatoes are soft, about 15 minutes. Stir in spinach (or chard) and continue to simmer until the greens are tender, about 10 minutes more. Puree the soup with an immersion blender or regular blender (in batches), leaving it a little chunky if desired. (Use caution when pureeing hot liquids.)
Serve the soup garnished with nutmeg, if desired, and topped with the croutons.