Preheat the oven to 400°F and line two sheet trays with aluminum foil.
In a large bowl, toss the sweet potatoes, onions, 3 tablespoons of olive oil, and rosemary to combine; season with salt and pepper to taste. Spread the mixture evenly onto the sheet trays and roast for 30 to 35 minutes or until tender, stirring once or twice while cooking. Let cool slightly.
Toss the sweet potato mixture in a large bowl with the Parmesan. In a separate bowl, whisk together the Balsamic vinegar and the 2 remaining tablespoons of olive oil. Toss the arugula with the vinaigrette and add to the sweet potato mixture. Toss lightly to combine. Sprinkle with toasted pine nuts (if desired) and additional grated Parmesan.
MAKE AHEAD: The sweet potato mixture can be roasted and tossed with Parmesan a day in advance. Just before serving, reheat the mixture in a 375°F oven for 10 minutes (or until warm) then toss with the arugula mixture and pine nuts.