Add the potatoes to a pot of boiling water and cook for 5 minutes.
Drain the potatoes and add them to a bowl with the onions and olive oil; toss to coat.
Line the grill with Reynolds Wrap® Heavy Duty Aluminum Foil. Add the potatoes and onions and cook for 8 to 10 minutes or until the potatoes and onions are browned and cooked.
Chill the potatoes and onions for 30 minutes until cool.
Add the cooled potatoes and onions to a medium-sized bowl and mix them with the celery, eggs, mayonnaise, mustard, salt and pepper.