Remove stems and roots from beets. Drop beets into boiling water; cook until tender. Drain and cool. Peel beets. With a melon ball scoop, remove the beet flesh, leaving a 1 cm shell. Heat oil in a large skillet. Saute kohlrabi, olives, pickles and onions until the vegetables are soft. Add lemon juice, salt and pepper. Remove mixture from heat; cool. Blend in the cooked rice, thyme and parsley. Stuff beets with this mixture and cover each with a slice of lemon. Bake in a preheated 160ºC oven for 35 minutes.