Preheat oven to 400 degrees F. Pierce sweet potatoes with a fork on all sides. Rub with 2 tsp olive oil and wrap each potato in foil. Bake for 1 hour or until tender. Let cool.
Process sweet potatoes in a food processor until smooth.
Heat oil in a Dutch oven over high heat. Add mushrooms, onion, 1 1/2 Tbs sage and garlic. Saute for 5 minutes. Add rice and cook for 1 minute, stirring frequently.
Stir in 1 cup sweet potato puree, 2 cups stock and 1/2 tsp salt. Bring to a boil then reduce heat to medium-high and simmer, stirring frequently for 8 minutes or until the liquid is reduced by half.
Add remaining stock and 1/2 tsp salt. Cook 25 minutes or until rice is tender while stirring frequently.
Remove from heat, stir in remaining sweet potato puree, parsley, pepper and mascarpone.
Divide among 6 serving dishes and top with sage and Parmesan.