Can I cook potatoes that have been halved and then wrapped in foil and left on the countertop for 8-10 hours on a hot summer day?

No! We receive many similar questions about prepared potatoes, asking  if they are safe when left out of the safe temperature holding  zones (‘safe’ is cooked potatoes either held at 140 degrees F serving temperature after preparation -  OR prepared potatoes, cooked then allowed to cool briefly and then held in refrigeration) . Wrapped or otherwise, if cooked potatoes are left at room temperature for longer than two hours then discard and do not consume. Food safety is imperative, so it’s always best to err on the side of caution.