Here's one spot where technique really makes a difference. Cook the onion until deep Brown and caramelized. It adds another layer of complex sweetness that differs from the Maple syrup. If you have some grated pecorino Romano it will make the puree richer and give it a nutty undertone.
Put the sweet potatoes, 1/4 cup Maple syrup, 2 tablespoons olive oil, 2 teaspoons salt and 1 teaspoon pepper in a large mixing bowl. Toss to evenly coat and transfer to a baking sheet. Roast in the oven until the sweet potatoes are soft, 35 to 40 minutes. Check on them after 15 minutes and stir, if needed.
Meanwhile, melt the butter and 2 tablespoons of olive oil together in a large skillet over medium-high heat. Add the sliced onions and 2 tablespoons maple syrup and season with salt and pepper to taste. Cook, stirring frequently, until the onion is deep Golden Brown, about 10 minutes, adding water, a tablespoon at a time, if the skillet gets too dry. Remove from the heat and set aside.
Transfer the sweet potatoes to a food processor and add a tablespoon of hot water. Pulse until just blended and then add half the caramelized onion. Pulse a few more seconds to incorporate the onion. For a very smooth puree, add another tablespoon of water and run the machine for one minute. Fold in the remaining caramelized onion, reserving some for garnish. Serve in warm dish and garnish with reserved caramelized onion. Serves four