Peel, wash and cut the kohlrabis into as fine slices as possible. Keep some tender kohlrabi leaves. Bring 300 ml salted water to the boil. Cook the kohlrabi slices for 4 minutes. Then drain and store the vegetable water.
Peel and wash the potatoes and cook them in salted water for about 20 minutes. In the meantime melt the fat. Sweat 1 tablespoon of flour in it. Deglaze with the vegetable water and cream while stirring constantly and bring to the boil.
Season to taste with salt, pepper and lemon juice. Alternately cover kohlrabi slices with ham and cheese. If possible use 3 kohlrabi slices each. Cover the upper slice with a tender kohlrabi leaf.
Turn filled kohlrabi one after the other in about 100 g flour, whisked egg yolk and breadcrumbs. Fry in hot oil at medium heat while turning for about 8 minutes. Season with salt and pepper. Cut tomatoes into fine strips.
Press the potatoes through a potato ricer and arrange them in portions with some tomato strips, kohlrabi cordon bleu and sauce. Serve garnished with fresh herbs and lemon.