Preparation Preheat oven to 425°F. Butter large rimmed nonstick baking sheet. Place onion in large bowl. Toss potatoes with 1/2 teaspoon salt in medium bowl. Let stand 5 minutes. Using hands, squeeze out excess liquid from potatoes. Add potatoes, 1/2 teaspoon salt, and melted butter to onion. Toss to coat. Divide mixture into 4 mounds on prepared baking sheet, spacing apart. Roast 15 minutes, then turn mounds over with spatula, pressing down to flatten to 4-inch-diameter rounds (cakes will still be soft). Reduce oven temperature to 350°F; bake until cakes are golden and crisp around edges, about 45 minutes longer.
Preparation Preheat oven to 425°F. Butter large rimmed nonstick baking sheet. Place onion in large bowl. Toss potatoes with 1/2 teaspoon salt in medium bowl. Let stand 5 minutes. Using hands, squeeze out excess liquid from potatoes. Add potatoes, 1/2 teaspoon salt, and melted butter to onion. Toss to coat. Divide mixture into 4 mounds on prepared baking sheet, spacing apart. Roast 15 minutes, then turn mounds over with spatula, pressing down to flatten to 4-inch-diameter rounds (cakes will still be soft). Reduce oven temperature to 350°F; bake until cakes are golden and crisp around edges, about 45 minutes longer.