Preheat oven to 375°F. Line a baking sheet with parchment paper.
Stir the oil, maple syrup, and sesame oil together in a small bowl.
Place the rice into a small saucepan and cover it with water. Let it soak for 20 minutes.
While the rice soaks, peel sweet potato and cut it lengthwise into strips that are just under ½ inch thick, as if you were making sweet potato fries. Arrange the strips on the baking sheet and rub them with the oil and maple syrup mixture. Arrange the strips on the baking sheet. Bake the sweet potato for about 25 minutes, until tender, gently turning the pieces halfway through.
When the rice has finished soaking, drain it, then return it to the pot. Add the 1 ⅓ cups of water, rice vinegar, salt, and sugar. Stir, then place the pot over medium heat. Bring the liquid to a boil, then reduce the heat, cover the pot, and let it simmer for 20 minutes, or until all of the liquid is absorbed.
Remove the pot from the heat and let the rice sit, covered, for 10 minutes, then remove the lid and fluff the rice with a fork.
Place a bamboo sushi mat onto a work surface and fill a small dish with water for wetting your hands. Place one of your sheets on the bamboo mat. Using wet hands, cover nori with a thin layer of rice. Arrange a third of your sweet potato strips in a single line along the width of nori, about one inch away from you. Arrange a few scallion pieces right alongside the sweet potato strips.
Take the bamboo mat and the end of the nori closest to you and tightly roll it over your fillings. Tuck the end of the nori in and continue rolling, using the mat to press your roll tight (see Note). Once completely rolled, use a large, sharp knife to slice the roll into 8 pieces. Wipe the knife clean with a wet cloth between cuts.
Repeat using your remaining nori sheets and fillings.
Sprinkle with sesame seeds and serve with soy sauce, wasabi and pickled ginger.