Trim and peel kohlrabi; quarter. Combine broth, fennel, and thyme in a saucepan; bring to a boil. Add kohlrabi, lower heat, and simmer, partly covered, until tender, about 15 minutes.
Drain and reserve broth. Arrange kohlrabi in serving dish and set in warm oven.
In a small heavy saucepan, blend together arrowroot with a tablespoon of broth. Add egg a