In a pot large enough to hold the potatoes, sweat butter and onions over medium heat, stirring occasionally, until translucent, about 5 minutes.
Remove from heat and add potatoes, mixing until coated with butter. Season to taste with salt and pepper.
Place parchment paper in bottom of 10" cast iron skillet. Layer potatoes and onions in skillet, then top with another piece of parchment paper and foil.
Bake at 400F for 1 hour 35 minutes, or until a skewer passes through potatoes with no resistance.
Remove foil and top parchment paper and broil for 10 minutes.
Remove skillet from oven and let cool. Then gently press down potatoes with a spatula to flatten and cool in refrigerator, wrapped with plastic wrap, at least 5 hours until solidified.
When ready to serve, coat a baking sheet with 3 tbsp melted butter. Cut around edge of skillet to loosen potatoes then invert onto the baking sheet. Cut potatoes into 12 wedges and bake until golden brown, 12-15 minutes.