This rustic vegetarian soup is perfect for winter nights.
Heat oil and butter in a large, heavy saucepan over low heat. Add leeks and stir well. Cover and cook until soft, about 5 minutes.
Add stock and bring to a simmer over medium-high heat. Add salt and pepper to taste. Add potatoes and bring back to a simmer. Cook until tender but not mushy, 10 to 15 minutes. Turn off heat.
Ladle about 3 cups of soup into a blender (don't fill more than half full). Hold the lid on with a dish towel to prevent splattering, and blend until smooth. Return pureed soup to saucepan, stir to combine, and bring back to a simmer. Season to taste.
Stir in greens. Cook for 2 to 3 minutes, until wilted and bright green, and serve immediately.
If you want to make the soup ahead of time, prepare through step 3. Reheat and stir in the greens just before serving.
