Sweet Potato and Yukon Gold Bake

olive oil

cups sliced onions (1/8 to 1/4-inch slices)

 teaspoon fresh thyme, minced

salt

 pounds of sweet potatoes, mixed in color if available, peeled and sliced into 1/8-inch thick rounds

 pounds Yukon Gold potatoes, peeled, and sliced into 1/8-inch thick rounds

 Tbsp melted butter

 teaspoon of salt

black pepper

Sprigs of fresh thyme