Penetrate potato on the top and through the sides...do not penetrate the bottom. Mix ingredients together and stir in 2 cups of water. Place potato in brine and freeze individually overnight.
Preheat charcoal grill.
Run water over potatoes in bag just enough to remove from ice. Slice 1-inch slice from middle of onion. Place onion in foil with the potato on top of onion. Top with heavy tablespoon of butter. Wrap foil and place on coals until done. Leave on coals for about 30 minutes or until determined texture.
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