Wash the potatoes and slice them into uniform ⅛-inch rounds. Peel them first if desired.
Peel, halve, and thinly slice the yellow onions.
Place 2 tablespoons of butter in a 9×13-inch baking dish and melt in the oven for two to three minutes.
Add the sliced onions, toss with the butter, and bake uncovered for 12 to 15 minutes, stirring halfway through.
Melt the remaining 4 tablespoons of butter. Toss the sliced potatoes with the melted butter in a large bowl.
Remove the baking dish from the oven. Add the potatoes and gently combine them with the softened onions. Spread into an even layer.
Cover tightly with aluminum foil and bake for 35 to 40 minutes, until the potatoes are nearly tender.
Remove the foil, gently turn some of the top slices, and continue baking uncovered for 20 to 25 minutes, until tender and golden brown.
Remove from the oven and allow the potatoes to rest for five minutes before serving warm.