Place a grill grate over your campfire.

Lay a thick piece of aluminum foil onto a flat work surface. Pile potatoes and carrots into the middle of the foil. Put butter on the vegetables and sprinkle onion soup mix over the top.

Fold opposite ends of the foil so they meet; roll together to form a seam. Roll remaining ends together to seal contents into a packet.

Cook packet on the grill until the potatoes and carrots are completely tender, about 45 minutes.