In a large pot or Dutch oven, bring the water to a gentle simmer.
While it heats, prepare your vegetables. Thoroughly wash the spinach leaves and remove any tough stems.
Peel the potatoes and chop them into a ½-inch / 1. 5 cm dice.
Add the spinach, diced potatoes, and half of the sliced green onions to the simmering water.
Season generously with salt and pepper.
Reduce the heat to low, cover, and let the soup gently simmer for 20-25 minutes, or until the potatoes are very tender.
To prevent curdling, temper the sour cream. Ladle about a cup of the hot soup broth into a small bowl with the sour cream.
Whisk until smooth and creamy, then pour this mixture back into the main pot, stirring to combine.
Stir in the remaining green onions and 1½ tablespoons / 22 ml (0.7 fl oz) of the lemon juice.
Let the soup warm through gently over low heat for another 5 minutes, but do not let it boil.
Taste the soup and adjust the seasoning.
Add more salt and pepper if needed. For a brighter, tarter flavor, stir in the remaining ½ tablespoon / 8 ml (0.3 fl oz) of lemon juice.
Serve immediately while hot, or chill completely to serve cold.
Garnish with an extra dollop of sour cream if desired.
