How to Make It
Place sweet potatoes in a food processor; process until smooth.
Heat oil in a Dutch oven over high. Add mushrooms, onion, 1 1/2 tablespoons sage, and garlic; sauté; 5 minutes. Add rice; cook 1 minute, stirring constantly. Stir in 1 cup sweet potato puree, 2 cups stock, and 1/2 teaspoon salt; bring to a boil (reserve remaining puree). Reduce heat to medium-high; simmer 8 minutes or until liquid is reduced by half, stirring frequently.
Add remaining 3 cups stock and 1/2 teaspoon salt; cook 25 minutes or until rice is tender, stirring occasionally. Remove pan from heat; stir in reserved sweet potato puree, parsley, pepper, and mascarpone. Divide risotto evenly among 6 shallow bowls; sprinkle with remaining 1 1/2 teaspoons chopped sage and Parmesan cheese.