1 lb. leeks (white and pale green parts), well rinsed and chopped
2 medium russet potatoes, peeled, cut into 1/2-inch dice
3 to 4 cups vegetable broth
2 tsp. grated fresh ginger
Salt to taste
1 tsp. fresh lemon juice
4 thin lemon slices, seeded
In large pot, heat oil over medium heat. Add onion and cook over low heat, stirring occasionally, until golden brown, about 20 minutes. Add leeks, potatoes, 3 cups broth and ginger. Bring to boil. Cover, reduce heat and simmer until potatoes are tender, about 20 minutes. Remove from heat; let mixture cool slightly, about 10 minutes.
Using slotted spoon, transfer all solids in saucepan to food processor. Process until very smooth, stopping to scrape down side of work bowl as necessary. With machine running, gradually add liquid in pan to processor. Return soup to saucepan. Bring to simmer, adding remaining broth for desired consistency. Season with salt and stir in lemon juice. Ladle into bowls. Garnish each with lemon slices and green onions if desired.