Step 1 Preheat oven to 425 degrees. Arrange the sweet potatoes on a foil-lined baking sheet and roast, turning occasionally, until soft, about 45 minutes. When cool enough to handle, remove and discard the peels and mash the potato flesh in a large bowl until just chunky. Stir in the bread crumbs, walnuts, sugar, mace, egg, three-fourths teaspoon salt and one-fourth teaspoon pepper until thoroughly combined.
Step 2 Place the flour in a wide, shallow dish. Using floured hands, form sweet potato mixture into 12 3-inch patties. (Each patty will use a scant one-third cup of the sweet potato mixture. Make smaller patties, if you like.) Coat each in flour, shaking off any excess, and transfer to a baking sheet. Heat one-fourth cup of the oil in a large skillet over medium heat. Working in batches, fry the patties, turning once, until deep golden brown all over, 8 to 9 minutes total. Transfer the fried patties to a paper towel-lined baking sheet to drain. Add additional oil, as needed, to the pan so the patties do not stick as they fry. Set aside when done.
Step 3Clean out and return the skillet to stove. Melt the butter in the skillet over medium-high heat then add the shallots with one-fourth teaspoon salt and a pinch of pepper. and cook, stirring often, until softened, about 5 minutes. Add the mushrooms and 1 tablespoon of the sage and cook, stirring often, until the mushrooms have softened to tenderness and most of the liquid has been absorbed, about 10 minutes. Add the cream and simmer, stirring often, until just thickened, 3 to 4 minutes. Remove the skillet from heat and stir in vinegar and season additionally to taste. (Recipe can be prepared to this point up to 24 hours in advance and kept, covered tightly, in the refrigerator.)
Step 4Bring the sweet potato cakes to room temperature and heat the ragout. Serve the sweet potato cakes with the hot mushroom ragout spooned over the top. Garnish with remaining 1 tablespoon sage and serve.