When the fire is lit but not ready, lay out 8 pieces of foil (about 12 by 8 inches) and brush with olive oil.
Shingle an eighth of the potatoes in a circle in the center of each piece of foil. Break the onion slices into rings and scatter evenly over each potato ring. Equally divide the remaining olive oil, rosemary, salt, and pepper over the potatoes.
With the potato rings intact, bring the two large edges of foil to the center, sealing firmly.
Fold over the other two short edges to seal. Place on a medium or medium-hot fire for 20 to 25 minutes or until the potatoes are cooked through (Cooking in foil packets is not an exact science, so remember to check one packet after 20 minutes for doneness and to make sure the bottom isn't burning.
If necessary, finish cooking with another foil sheet under the packets.).