Peel, wash and slice the kohlrabi. Pre-cook in boiling salted water (briefly) firm to the bite. Do the same with the potatoes.
Peel the onions and cut into fine rings. Cut the salami into strips or small pieces.
Layer these 3 ingredients alternately in a gratin dish (the lower and upper layers should consist of potato slices). Sprinkle the kohlrabi and potato layers with a little grated cheese and season with salt, pepper and a little nutmeg.
Beat the milk (or the milk-cream mixture) with 2 eggs and season with salt, pepper and paprika powder. Then pour over the casserole.
Mix the remaining cheese with the breadcrumbs and sprinkle over the vegetables. Melt the butter and drizzle over it. Bake at 200 ° C (top / bottom heat, preheated) for about 40 - 45 minutes.