Set the oven to broil with a rack set about 4 inches from the heat source.
Peel the zest away from the lemon in long strips, avoiding the white pith. Place the lemon zest, rosemary sprigs and oil in a small saucepan over medium-low heat and heat until the oil just comes to a simmer, about 10 minutes. Turn off the heat and allow to sit at room temperature while you prepare the soup.
Melt the butter over medium heat in a large saucepan. Add the white and light green parts of the scallions (about 1/2 cup) and the garlic. Cook until softened, stirring constantly, 3 to 5 minutes. Add the chicken broth, 2 cups water and the French fries, bring to a boil and then reduce to a simmer. Simmer uncovered until the potatoes soften and start to break apart in the broth, about 10 minutes.
Meanwhile, strain the rosemary and lemon strips from the oil. Juice the lemon and whisk into the strained oil. Season with salt and pepper. Brush some of the oil onto the halved baguette, place on a baking sheet and broil until lightly golden brown, about 3 minutes.
Once the potatoes are fully softened, use an immersion blender to puree the soup until completely smooth. Season with additional salt if needed.
To serve, ladle the soup into bowls. Top with a dollop of sour cream. Drizzle with some of the remaining rosemary-lemon oil and garnish with the dark scallion greens. Cut the baguette into smaller pieces and serve alongside the soup.
If using "crinkle-cut" fries, add enough water to achieve a creamy, but not too thick, consistency.