Preparation

• Position a rack in the center of the oven and heat the oven to 425°F. Line a rimmed baking sheet with foil. Prick the sweet potatoes all over with a fork, transfer to the baking sheet, and bake until fork-tender, about 1 hour, turning halfway through. Let cool slightly.
• Meanwhile, combine 3 Tbs. of the maple syrup, 1/8 tsp. of the cinnamon, and 1/8 tsp. salt in a small skillet. Add the walnuts and stir to coat. Cook over low heat, stirring constantly, until the walnuts begin to darken, about 2 minutes. Spread on a plate to cool, then chop coarsely.
• Halve the sweet potatoes lengthwise and scrape most of the flesh into a large bowl, leaving 1/8 inch to 1/4 inch of it in the skins. Set the skins aside on the baking sheet. To the flesh, add half of the cheese, the remaining 1/4 cup maple syrup, 1/4 tsp. cinnamon, 1 tsp. salt, and 1/2 tsp. black pepper. Blend with a fork or potato masher until smooth. Divide the mixture evenly among the skins, and bake until golden brown, about 15 minutes.
• Top with the remaining cheese and the nuts, and serve.