A very easy and delicious meal to make on a campfire. Make sure your boneless loin has at least 1/4 inch of trim for moisture. I have put in instructions for using foil to cook the dish, but making it in a heavy cast iron vessel is easier.
Figure out how much foil you will need to totally surround the loin and the vegetables.
Lay this amount flat and taking another piece the same size, triple fold it to make the bottom of your "pan". Lay this in the center of the first piece. Place the loin and vegetables on the foil, pull the sides of the long piece upwards. Sprinkle rosemary all over the meat and vegetables.
Cut the butter into chunks and place on top of the loin.
Finish pulling the foil around the ingredients, folding the ends inches.
Seal the foil envelope as well as you can and place it on the rack above the flames.
Keep an eye on it, moving it around from spot to spot to keep it out of extreme heat.
This will take anywhere from 3 to 6 hours to cook, depending on the heat of your fire.