At Home: $4 per person. Use bacon, which costs less than guanciale.
In a soup pot, heat 1 tablespoon of the oil. Add the bacon and cook over moderately high heat until crisp, 5 minutes. Add the onion and leek and cook over low heat until softened, 10 minutes. Add the wine and simmer until nearly evaporated, 5 minutes.
Tie the parsley sprigs, thyme sprigs and bay leaf with string; add to the pot along with the potatoes and stock. Bring to a boil, then simmer until the potatoes are tender, 10 minutes. Add the cabbage, season with salt and pepper and simmer until the cabbage is tender, 10 to 15 minutes longer. Discard the herb bundle. Stir in the chives.
Meanwhile, in a medium skillet, heat the remaining 2 tablespoons of oil. Add the bread and garlic and cook over moderate heat, stirring, until the croutons are golden brown, 6 minutes; discard the garlic. Season with salt and sprinkle with the parsley.
Ladle the soup into deep bowls. Sprinkle with the croutons and serve.