Cut large pieces of aluminum foil (one for each potato), then spray or brush liberally with the oil. Set aside.
Peel and slice the onion evenly.
Clean the potatoes well. Then carefully slice them into 1/4 inch slices (about 0.5-0.6 cm). I recommend using a very sharp knife or a mandoline for this so you get an even thickness.As you cut the potatoes, lay them out on the oiled foil. Don't crowd them - this will keep them crispy on the grill.
Add 1/5 of the onions and 1 Tbsp of butter and season with salt and pepper. Fold the foil tight around the potatoes.
Add the potato packs to a medium-high heat grill and allow to cook for 30-40 minutes. Flip the potatoes halfway through - this will make them deliciously crispy with a tender inside.
Remove carefully from the grill and enjoy.
Video
Notes
Storage: These potatoes are best enjoyed fresh off the grill. You can store any leftovers in an airtight container in the fridge for 2-3 days.
Make ahead: Prepare the potato packets a day in advance and keep them in the fridge until ready to grill.
Using an oven: Alternatively, you can bake the potato in foil in the oven at 375ºF/190ºC for about 40-45 minutes. The potatoes should be cooked through.