Melt butter in soup pot over medium heat; add leek, potatoes and garlic. Cook for several minutes, then add greens and herbs; season with about a teaspoon of salt. Add the liquid, stir and bring to a boil. Turn down heat and simmer, partially covered, about 20 minutes (until the potatoes are tender).
Purée the soup, add a squeeze of lemon and adjust salt and pepper to taste.
In a skillet, melt butter over medium-low heat. Toss in bread cubes and cook until crisp and golden.
Divide soup among bowls, swirl in cream and top with croutons.