Bring the stock, 300 ml milk, some salt and pepper to the boil in a saucepan. Clean and peel the kohlrabi, setting aside the small leaves. Halve the kohlrabi and cut into thin slices.
Add the kohlrabi to the boiling milk, cover and simmer over medium heat for about 7 minutes. In the meantime cut ham into strips. Wash the kohlrabi leaves, shake dry and cut into strips, up to 3 small ones for garnishing.
Grease 3 ovenproof dishes (approx. 350 ml; 14 cm Ø). Remove kohlrabi from the milk with a skimmer and place in a bowl. Stir the sauce thickener into the kohlrabi stock. Bring to the boil while stirring.
Stir 30 g cheese, ham and kohlrabi leaves into the sauce, heat and season with salt and pepper. Fold the kohlrabi into the sauce, then distribute into the 4 moulds and sprinkle with the remaining cheese.
Gratinate under the preheated grill or in the preheated oven (electric cooker: 250 °C/ circulating air: 225 °C/ gas: level 5) for approx. 8 minutes. Bring 375 ml water and 1/2 teaspoon salt to the boil in a saucepan. Remove from the heat, add 125 ml cold milk and stir in the puree flakes with a wooden spoon.
After about 2 minutes add butter and stir again briefly. Garnish finished kohlrabi ham gratins with leaves. Add mashed potatoes separately.