Wash the meat and dab dry. Heat oil in a pan. Brown the meat on all sides over medium heat for 8-10 minutes, remove and let it cool down. Take the puff pastry out of the fridge about 10 minutes before processing.
Clean the cabbage, remove the outer leaves. Cut cabbage into quarters, cut out the stalk. Cut cabbage into strips, wash and drain. Line a baking tray with baking paper. Coat the edges of the pastry with water.
Place the meat with the upper side on the dough, beat the ends together, carefully turn everything over and place it on the baking tray. Lightly scratch the top of the dough several times with a sharp knife.
Whisk the egg yolk and 3 tablespoons of water, brush the dough with it. Cook in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 45-55 minutes, cover with foil if necessary. Peel and chop the onion.
Heat butter in a large pot. Sauté diced onions in it. Add savoy cabbage, season with salt and pepper and fry for about 10 minutes. Deglaze with stock and whipped cream, finish cooking for another 5 minutes, season with salt, pepper and sugar.