Drain the beans on a sieve. Clean, wash and cut the Chinese cabbage into strips. Wash, peel and thinly slice or slice potatoes. Layer vegetables and potatoes in a flat, ovenproof dish in flakes.
Peel onion and garlic. Chop the onion finely, press the garlic through a garlic press. Wash the sage, dab dry and chop, except for a few leaves for garnishing. Mix sour cream, milk, egg, onion, garlic, sage, salt and pepper.
Pour over the gratin and sprinkle with cheese. If necessary, spread a few flakes of fat on the gratin and bake in a preheated oven (electric range: 200 °C/ gas: level 3) for 45-50 minutes. Sprinkle the finished gratin with coloured pepper and serve garnished with sage.