Combine broth and potato cubes in a small saucepan and simmer, covered, for approximately 10 minutes or until potatoes are tender.
Drain the potatoes, saving the broth, and puree with a food mill or through a fine-meshed sieve. (Don't use a blender or food processor as potatoes will become very sticky.) Stir the broth into the potato puree and set aside.
In large pot, heat butter and oil over medium heat. Add the onion, leeks, and scallions and cook, covered, for approximately 5 minutes or until tender, stirring occasionally.
Add 1/2 inch water and carrots and simmer for five minutes. Add the chard stems and cook them, covered until tender (approximately 3-5 minutes). Add the chard leaves and cook for 10 minutes, stirring occasionally.
Add the potato puree, stir thoroughly, and cook until heated through. Add salt, lemon juice, and black pepper and mix thoroughly.
Garnish with parsley if desired, and serve immediately.