Not really an open fire thing, but here is what I do on a grill (could be over a fire).
Ingredients/supplies:
Medium sized red potatoes
Sliced Onion (I like sweet onions like vidallia)
Butter
Salt
Foil
Method:
Arrange a double layer of tin foil slightly longer than your grilling surface on your workspace.
Place first sliced potato about 2" from the end of foil.
Separate each potato slice and insert a few bits of the sliced onion.
Continue until you are about 2" from other end of foil (for me about 5 spuds)
Put 3 or 4 pads of butter evenly spaced on the potato/onion.
Salt to taste
Pull sides of foil up together and roll back down onto spuds.
Push the fallen-over slices at the end of the roll up with a finger and roll each end up tight.
Grill 40 minutes on a medium heat, turning 1/4 turn every 10 minutes.
Make extra tubes and re-heat them to eat with your eggs in the morning...
When you get this right and the potato just starts browning and the onions caramelize, it is heavenly (that's the Irish in me talking...)