Step 1Heat the oil in a medium to large (at least 4-quart) stock pot over medium heat and cook the carrots and leeks until tender, about 10 minutes. Add the garlic and saute an additional minute until fragrant but not browned.
Step 2Add the chicken broth, potato cubes and the chopped kale, mustard and collard greens; stir.
Step 3Bring to a boil, then reduce the heat to a simmer. Cover partially and simmer 1 hour. Remove from heat and puree with an immersion blender or in a blender or food processor until smooth.
Step 4Return the soup to the heat and simmer, uncovered, until it thickens slightly, about 5 to 10 minutes. Skim off any foam. Stir in the cream, and simmer until slightly thickened, about 10 minutes.
Step 5Season with white pepper. Taste and add salt if desired. Ladle into bowls and garnish each with 3 or 4 small pieces of lemon.