Add leeks and onion; sauté until softened, about 5 minutes. Add garlic; sauté 2 minutes. Add English spinach, silverbeet, potato and carrot. Stir in stock, water and salt. Simmer, partially covered, for 50 minutes.
Purée soup in batches in a blender or food processor. Return to pan. Stir in sour cream. Heat just until warmed.