Preheat oven to 400 degrees F.
Combine melted butter and crushed rosemary in a small bowl. Pour 2 tablespoons of that mixture in the bottom of a 2-quart baking dish. Arrange potato slices vertically in the dish. Add a sliver of onion between every few slices of potato. Brush top with remaining butter mixture. Season generously with salt and pepper.
Cover dish with foil and roast for 1 hour, covered, until potatoes are tender.
Increase oven heat to 450 degrees F. Remove foil and roast another 10-15 minutes, until tops of potatoes are browned and crisp.