Peel and wash the potatoes. Cut into thicker slices. Skin sausage, cut into slices. Layer both in flakes in a deep baking tray (approx. 25 x 35 cm) or deep fat pan or large casserole dish (approx. 3 1/2 l capacity) greased with 1/2 tbsp. oil.
Peel onions and garlic. Cut both finely. Fry in 1 tbsp. hot oil. Stir in milk, cream and sour cream, bring to the boil. Stir in mustard. Season to taste
Pour the icing over the potatoes. Bake in the preheated oven (electric cooker: 175 °C/circulating air: 150 °C/gas: level 2) for approx. 1 1/4-1 1/2 hours. Coarsely grate the Gouda, sprinkle over the potatoes after about 50 minutes