How to Make It
In a large saucepan, cover the beets with cold water and bring to a boil over hgih heat. reduce the heat to moderate and simmer the beets until tender, about 25 minutes. Drain the beets and let them cool slightly, then peel and cut into 2-inch chunks with a small sharp knife.
In a large nonreactive saucepan, melt the butter over moderate heat. Add the onion and thyme sprigs and cook, stirring, until the onion is translucent, about 4 minutes. reduce the heat to moderately low and add the garlic. cook until softened, about 5 minutes. Add the beets, chicken stock and a large pinch each of salt and pepper. increase the heat to moderate, cover and simmer for 10 minutes. remove from the heat, uncover and let stand for 5 minutes. Discard the thyme sprigs.
Working in batches, puree the soup in a blender until very smooth. Season with salt and pepper and let cool completely. Pour the soup into airtight containers and freeze for up to 1 month.
Roast the chiles directly over a gas flame or under the broiler, turning, until charred all over. transfer the chiles to a large bowl, cover with plastic wrap and let steam for 5 minutes. Peel the chiles and remove the seeds, then transfer to an airtight container and strain any accumulated juices on top. Coat the chiles with the olive oil, season lightly with salt and pepper and freeze for up to 1 month.
To serve, thaw the soup and chiles in the refrigerator overnight. rewarm the soup over moderate heat, stirring often. Finally chop the chiles and put them in a small bowl. Fold in the sour cream. Season the soup with salt, pepper and the lime juice and ladle into bowls. Dollop or swirl about 2 tablespoons of the chile cream in each serving.