Preheat a grill to medium heat.

Thinly slice the radishes carefully with a knife or mandolin to 1/8th of an inch thick. Slice the onion and mince the garlic.

Take two large pieces of foil and overlap the middle seam. SPray with non stick spray.

Season the potatoes with the salt, pepper and mix with the garlic and onion. Spread the mixture out thinly across the foil, leaving a lot of room for the foil to cover and seal the sliced radishes. Before wrapping, take the butter pieces and sprinkle them across the top of the sliced and seasoned radishes. Wrap securely.

Place over direct heat for about 4-5 minutes. Carefully flip the foil packet with oven mitts or tongs and cook for another 3-4 minutes on the other side. Remove from the heat and carefully open the packet to release steam. It will be very hot, so do this with caution

If you would like to brown the radishes a bit more, place them open faced in the foil back on the heat to remove some of the moisture or under a broiler for a few minutes in the oven.

Place in a bowl and serve.