Move a rack about 10 inches about your campfire flames.

Cut potatos and onions into quarters.

For the loin and vegetables, figure out how much foil you will need to cover them all.

Make the bottom of your "pan" by triple folding another piece of the same size that you have laid flat. Put this in the middle of the first piece. On top of the foil, put the meat and vegetables. Pull the sides of the long piece up. Put rosemary on the meat and vegetables all over them.

Put the butter on top of the loin after cutting it up. Finish wrapping the foil around the food and fold the ends in an inch or two. Put the foil envelope on the rack above the flames and seal it as well as you can. Make sure you keep an eye on it and move it around so it doesn't get too hot. Based on how hot your fire is, this could take anywhere from 3 to 6 hours to cook.