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1
Set the oven to broil with a rack set about 4 inches from the heat source.
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2
Peel the zest away from the lemon in long strips, avoiding the white pith.
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3
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.
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4
Turn off the heat and allow to sit at room temperature while you prepare the soup.
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5
Melt the butter over medium heat in a large saucepan.
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6
Add the white and light green parts of the scallions (about 1/2 cup) and the garlic.
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7
Cook until softened, stirring constantly, 3 to 5 minutes.
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8
Add the chicken broth, 2 cups water and the French fries, bring to a boil and then reduce to a simmer.
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9
Simmer uncovered until the potatoes soften and start to break apart in the broth, about 10 minutes.
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10
Meanwhile, strain the rosemary and lemon strips from the oil.
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11
Juice the lemon and whisk into the strained oil.
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12
Season with salt and pepper.
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13
Brush some of the oil onto the halved baguette, place on a baking sheet and broil until lightly golden brown, about 3 minutes.
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14
Once the potatoes are fully softened, use an immersion blender to puree the soup until completely smooth.
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15
Season with additional salt if needed.
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16
To serve, ladle the soup into bowls.
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17
Top with a dollop of sour cream.
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18
Drizzle with some of the remaining rosemary-lemon oil and garnish with the dark scallion greens.
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19
Cut the baguette into smaller pieces and serve alongside the soup.
