1. Melt the butter in a medium-size heavy kettle over medium heat. Add the leeks and cook, stirring, until translucent and tender, 10 to 15 minutes.
2. Stir in kohlrabi and potato. Cook, stirring, 3 to 4 minutes longer. Stir in stock and thyme. Simmer over low heat until the kohlrabi and potato are very tender, about 20 to 25 minutes.
3. Remove about half of the vegetables and puree in food processor or blender, adding stock if too thick. Return the puree to the pot. Stir in cream. Taste and season with salt and pepper. Simmer over low heat for 10 more minutes.
4. To serve, ladle soup into serving bowls and sprinkle liberally with scallions. Serve immediately with warm crusty bread. Makes 6 servings