4 tablespoons (1/2 stick) unsalted butter, melted, divided, plus more for pan
1/2 teaspoon caraway seeds
2 tablespoons rendered duck or bacon fat, melted
1 teaspoon (or more) kosher salt
1/2 teaspoon freshly ground black pepper
2 pounds medium Yukon Gold potatoes, unpeeled, cut into 1/8"-thick slices with a mandoline or V-slicer
1 small sweet onion (such as Maui), very thinly sliced
Ingredient info: Rendered duck fat can be found at better supermarkets and at specialty foods stores and dartagnan.com.
Special equipment: The outer ring from a 9"-diameter springform pan