Slice leeks crosswise and immerse in a large bowl of water to rinse off dirt; lift out to a colander, leaving dirt behind, and rinse again in fresh water if necessary. In a large saucepan over medium-high heat, melt butter. Add leeks and salt and cook until limp, stirring often, about 5 minutes. Add broth and potato. Bring to a boil, covered; reduce heat and simmer until potatoes are tender when pierced, 10 to 15 minutes. Add spinach and cook until wilted, about 1 minute.
Purée in a blender, in batches if necessary, holding lid down with a towel. Pour back into saucepan and thin with more broth if you like; reheat until hot. Season with salt and pepper. Divide soup between 4 serving bowls and top each with about 1/4 cup croutons.
Note: Nutritional analysis is per serving.