Remove the tough outer peel from the kohlrabi and grate with a food processor or box grater. Or chop the cabbage into shreds. Place into a large bowl.
In a small bowl, combine all dressing ingredients except mayonnaise and stir to dissolve the sugar and salt.
Add mayonnaise a tablespoon at a time until it reaches desired consistency.
Pour ¾ of the dressing over the kohlrabi (or cabbage) and stir to mix. It should barely coat the kohlrabi without a lot still in the bowl. Add the remaining dressing if needed.
Cover and chill for a few hours (or overnight) to allow the flavors to meld and the kohlrabi to disgorge some of its liquid. Stir and taste before serving. Add remaining dressing if desired and adjust seasonings as needed.
Notes
When my brother makes the dressing he only adds a few tablespoons of mayonnaise making it very thin. If you like your dressing thicker, add more mayo. But you may want to wait until after the salad has set for awhile before doing so.