Peel and cut the carrots into 1/2-inch rounds. On a rimmed baking sheet, toss the carrots with 2 tablespoons of the olive oil and sprinkle generously with salt. Set an oven rack 6 to 8 inches from the heat source and turn on the broiler. Broil the carrots until they brown and soften, turning them over with a spatula every 5 minutes or so; this should take 15 to 20 minutes.
Meanwhile, bring the stock to a boil, add the ginger and the sprig of thyme and simmer gently for 15 minutes.
Put the onion in a medium stock pot with the remaining olive oil. Brown the onion over medium heat, stirring frequently. Add the garlic, and then add the carrots.
Remove the ginger and thyme from the stock and add the stock to the pot with the onions and carrots. Bring to boil and simmer for 5 to 10 minutes, until the carrots are soft enough to puree.
Use an immersion or a standard blender to puree the mixture until smooth. If the soup seems too thick, add more stock or water and reheat gently. Add salt and pepper to taste. To serve, garnish with chopped fresh thyme.