Finely shred the red cabbage and coarsely grate the apple. Halve the onion and cut into thin strips.
Put all the ingredients for the red cabbage, except the onion and oil, in a bowl and knead vigorously by hand, seal airtight and leave to infuse for 30–60 minutes.
Squeeze the cabbage with your hand, collecting the juice. Heat the oil in a pot and sauté the onion in it until translucent. Add the cabbage and fry for 4 minutes. Pour in the squeezed juice and let it reduce for 15 minutes; season with salt, pepper and sugar; keep warm.
Meanwhile, for the dumplings, finely grate the 300 g of potatoes and immediately knead them with 1 teaspoon of salt so they don't turn brown; cover and let sit for 5 minutes.
Spread out a clean kitchen towel, place the potatoes on it, twist the towel and press the liquid out of the mixture, leaving about 150 g of grated potatoes.
With moistened hands, form four dumplings from the dough and place them in the water. Reduce the heat and let the dumplings simmer in the simmering water for about 25 minutes until cooked through.
Preheat the oven to 180 degrees top/bottom heat. planted.roast directly from the fridge (it must be cold) onto a baking tray lined with baking paper and bake in the preheated oven for 20 minutes.
Remove the gravy from the packaging and heat it briefly in a small saucepan. Stir evenly with a whisk to create a homogeneous sauce. Then turn off the heat and close with a lid to keep it warm.
Switch off the oven, take out the planted.roast and close it again so that it stays warm. Brush the planted.roast with the gravy on all sides (except the underside) and put it back in the oven for 10 minutes. Leave the oven door slightly ajar.
Remove the planted.roast and cut into slices with a sharp knife and serve immediately with dumplings and red cabbage.