Peel and slice the potatoes. Peel the sausage and also cut it into slices. Layer both tiles in a baking dish.
Peel onions and garlic and cut into cubes or press through, then fry in hot oil. Deglaze with milk, cream and sour cream and bring to the boil. Season well with mustard and the spices. Pour the glaze over the potatoes and put in the oven. It should be preheated to 150 degrees (circulating air). In total, the gratin stays in the oven for 1 1/4 to 1 1/2 hours. After 50 minutes, the grated Gouda is sprinkled over it.