Preheat oven to 375°.
Spread out the carrots and onions on a baking sheet. Drizzle them with the olive oil and sprinkle with salt. Roast for 45 minutes, until the carrots and onions are soft and beginning to brown around the edges.
In a medium (4- to 5-quart) soup pot or Dutch oven, combine the roasted vegetables with the chicken broth. Bring up to a simmer.
With an immersion blender or in a traditional blender (in 2 batches), process the soup until smooth.
Ladle the soup into bowls and drizzle with the rosemary oil.