Preheat oven to 375 degrees. Coat bottom of baking dish with 2 tablespoons of melted butter. Place layer of pan with sliced potato medallions. Cover with layer of sliced onions. Sprinkle with salt and pepper and brush with melted butter. Continue layers of potatoes and onions finishing with layer of potatoes. Cover dish and bake for 30 minutes. Remove foil and finish baking 15 to 20 minutes until golden brown.
Variation: When grilling, make individual packages for each person. Brush melted butter over center of foil and layer the potatoes and onions with butter and salt & pepper. Seal foil and place on grill. Cook for approximately 15 minutes, turning package occasionally.