Wash the potatoes and cook covered in water for about 20 minutes. Grate cheese. Melt the fat in a pot. Briefly sweat flour in it. Deglaze with 3/8 litres of water and milk. Bring to the boil and stir in broth.
Let simmer for about 5 minutes while stirring. Stir in half the cheese and melt. Season to taste with salt and pepper. Drain, rinse and peel the potatoes. Put potatoes and sauce in an ovenproof dish.
Sprinkle the rest with cheese. Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for approx. 15 minutes. Wash the chives, dab dry and cut into small rolls, except for a little garnish.
Sprinkle potatoes with chives and garnish. Serve with a green salad.