Heat olive oil in a large skillet over medium heat. Add onions and garlic, sauté until fragrant.
Add sliced Portabella mushrooms and cook for about 5 minutes, until softened.
Stir in Arborio rice and cook for 2 minutes until the rice is lightly toasted.
If using wine, add it now and let it reduce for 1-2 minutes.
Gradually add warm broth one ladle at a time, stirring constantly until absorbed before adding more. Continue until the rice is tender and creamy, about 20-25 minutes.
Stir in the cream, Parmesan, and butter. Season with salt and pepper.
Serve hot with additional Parmesan if desired.
Notes
For a vegan option, swap the butter and cream for plant-based alternatives and use vegan Parmesan.