Preheat the oven to 350 degrees F. Use a small heart-shaped cookie cutter (available at kitchenware stores) to cut each slice of bread into 3 or 4 small hearts. Place the hearts on a non-stick or parchment-lined baking sheet. Brush the tops of the bread lightly with olive oil; sprinkle with the Parmesan cheese. Bake in the oven until lightly toasted, about 8-10 minutes; cool to room temperature.
Heat the oil for the soup in a pot over medium heat. Add the onion and garlic and cook 4 to 5 minutes. Stir in the flour. Slowly pour, mixing steadily, in the stock. Add the remaining ingredients and simmer the soup until the potatoes are tender, about 20 minutes. Puree the soup in a food processor; return to the pot and a simmer. Add a little extra stock if too thick; adjust seasoning. Pour into bowls; garnish with croutons and serve.