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Adjust rack to top position (middle and top positions for 4 servings) and preheat oven to 450 degrees. Wash and dry all produce.
Dice potatoes into ½-inch pieces. Halve and peel onion; grate one half on the largest holes of a box grater (grate all the onion for 4). Transfer to a large bowl. Peel and mince or grate half the garlic (keep remaining garlic whole). Strip rosemary leaves from stems; finely chop leaves until you have 1 tsp (2 tsp for 4). Trim green beans if necessary.