Preheat oven to 425°F. On baking sheet, toss mushrooms with ⅓ cup of the olive oil, then sprinkle with 1 tsp of the salt and ½ tsp of the pepper and add rosemary. Spread evenly and roast until browned and crisp, 13 to 15 minutes, tossing halfway through.
Meanwhile, in saucepan, bring broth to a simmer and keep warm over low heat, covered.
In large pot, heat remaining 3 tbsp olive oil over medium heat. Add onion and cook, stirring, until softened, about 4 minutes. Add rice and cook, stirring, until evenly coated and toasted, 1 to 2 minutes.
Add wine and cook, stirring, until absorbed, about 1 minute. Add 1 cup of the broth and cook, stirring constantly, until almost absorbed. Continue adding broth 1 cup at a time, stirring, until almost all absorbed between additions, about 15 minutes.
Stir in half of the mushrooms and continue cooking, adding more broth if rice is dry, until rice is tender and creamy, 3 to 5 minutes. Stir in butter, Parmesan, mascarpone, remaining ¾ tsp salt and remaining ½ tsp pepper.
To serve, divide risotto among bowls and top with reserved mushrooms, shaved Parmesan, herbs (if using) and more coarsely ground black pepper (if desired).