Place 1 tablespoon oil and 2 tablespoons broth in a large, non-stick pot over medium heat. Add the onions, leeks and marjoram and saute for 15 minutes, or until vegetables are tender.
Add the potatoes and 7 cups broth, reduce heat to low, cover and simmer for 25 minutes. Add the heavy cream, ground black pepper and cardamom. Puree the soup in small batches in a blender or food processor.
Place the mushrooms and the remaining 1 tablespoon of broth in a large skillet over medium heat and saute until the liquid evaporates and the mushrooms are golden in color. Season with salt and pepper to taste. Add the mushrooms, precooked carrots and precooked potatoes to the pureed soup. Stir together and serve.