Preheat the oven to 350 degrees F (175 degrees C).

Lay a large sheet of heavy-duty aluminum foil over a baking sheet. Place sauerkraut, sausage, onion, oil, butter, pepper, and caraway seeds in the center of the foil; mix until well combined. Bring up the sides of the foil and double-fold the top and ends to make a packet, leaving room for heat circulation inside.

Place the foil package and baking sheet into the preheated oven and roast for 30 minutes.

Remove from the oven; open the packet carefully by cutting along the top fold with a sharp knife. Allow the steam to escape, then open the top of the foil packet. Stir in green apples and reseal the foil, adding another piece to the top if necessary. Return to the oven for 10 more minutes.

Remove from the oven, season with salt, and serve.