Peel kohlrabies and scoop out centers. Place the vegetables in 1quart salted water and boil until half done, about 25 minutes, but the time depends on age of the kohlrabies. Drain kohlrabies and save the cooking liquid.
Chop very fine the scooped-out centers of kohlrabies and all the chopped leaves. Add sugar and sauté the mixture for a few minutes.
Add 2cups of the cooking liquid, and cook the chopped kohlrabi centers and leaves until about half done.
Soak the roll in the milk, squeeze roll, and chop or shred into small pieces.
Fry onion in butter in a separate frying pan for a few minutes. Add ground meat and sauté for 5 minutes.
Add chopped roll, the pepper, salt and the ½ egg. Stir well, and remove from heat. Adjust salt if necessary. Stuff this into the empty kohlrabi shells.
Place stuffed shells in a baking-serving casserole. Pour chopped leaf mixture around shells, and add the chicken broth if some extra liquid is needed.
Cook the kohlrabies over very low heat for 25 to 30 minutes, or until vegetables are done.
Make a thickening with flour and ¾tablespoon cold water. Take a ladleful of liquid from the casserole, whip it into this flour-water mixture, and pour it back into the casserole. Cook for 5 minutes longer.
Serve 1 stuffed kohlrabi as a hot first course, or two as a luncheon main course.