Preheat oven to 450°F (230°C). Bring potatoes to a boil in a large pot and cook, 5 min. Drain potatoes, rinse with cold water and cut in half lengthwise.
Place potatoes in a bowl and stir in carrots, parsnips, onion, maple syrup, rosemary, butter, oil, garlic, salt and pepper. Transfer vegetable mixture to a baking sheet, and bake on lower rack of oven for 20 min., stirring halfway through. Garnish with green onions.