Note: If you can’t find fresh marjoram, you can substitute it with equal parts fresh thyme and oregano.
Preheat oven to 450°F. Toss sweet potatoes with oil and marjoram in large bowl. Spread sweet potatoes in single layer on 2 baking sheets. Place 1 sprig marjoram atop sweet potatoes on each baking sheet; set 1 sheet on top oven rack and 1 sheet on bottom rack.
Roast sweet potatoes 10 minutes. Stir, switch position of baking sheets on oven racks, and roast 5 minutes more. Stir sweet potatoes again, and roast 5 additional minutes, or until browned on edges and beginning to brown evenly.
Sprinkle 2 tsp. garlic over each batch of sweet potatoes, and toss to coat. Roast 5 minutes more, or until sweet potatoes are browned and crisp. Transfer sweet potatoes to serving bowl, and season with salt and pepper, if desired. Sprinkle with Parmesan cheese, if using, and parsley. Garnish with parsley and marjoram sprigs.