Wash potatoes thoroughly and cook in boiling water for 20 minutes. In the meantime, peel, wash, halve and slice the kohlrabi. Put kohlrabi in boiling salted water and cook for eight to ten minutes.
Pour into a sieve and drain well. Drain the potatoes, rinse and peel. Cut potatoes into slices. Grate cheese. For the sauce melt fat, dust with flour, sweat on, gradually add stock and cream while stirring, bring to the boil, season with salt, pepper and sugar and stir in 75 grams of cheese.
Bring to the boil again briefly. Layer some sauce, 1/3 of the potatoes and kohlrabi and four slices of ham in a greased, ovenproof dish. Layer the rest in the same way, finishing with the potatoes, kohlrabi and sauce.
Sprinkle with 75 grams of cheese. Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for 30 minutes. Remove from oven and garnish with basil.