Set the oven to 400°F. Line a large baking sheet or shallow baking pan with parchment paper and lightly brush it with some of the melted butter.
Arrange the potatoes: Place the potato rounds in a single layer on the prepared baking sheet.
Add butter: Lightly brush the tops of the potato slices with the remaining melted butter.
Add onion: Sprinkle a small amount of chopped sweet onion over each potato round.
Top with cheese: Cover each potato round with shredded Swiss cheese.
Bake: Place in the oven and bake for 22–28 minutes, or until the potatoes are fork-tender, the onions are softened and lightly browned, and the cheese is melted with golden spots.
Rest: Allow the potato bites to cool for 3–5 minutes.
Serve: Transfer to a serving dish and serve warm.