Place potatoes in saucepan with enough water to cover; bring to boiling. Reduce heat, cover, and simmer for 20 to 25 minutes or just until potatoes are tender; drain well. Cool slightly. Cover and chill at least 2 hours or up to 24 hours. Cut chilled potatoes into 1/2-inch-thick slices.
Heat oil over medium heat in a heavy 12-inch nonstick skillet. Add potatoes, onion, salt, and pepper. Reduce heat to medium; cook for 20 to 25 minutes or until browned and crisp, carefully turning potatoes frequently. Add rosemary and cook 2 to 3 minutes more.