Sauté aromatics: In a large pan over medium heat, add olive oil. Cook onions until translucent (about 5 minutes), then stir in garlic for an additional minute.
Cook mushrooms: Add sliced mushrooms and sauté until golden brown (about 8 minutes).
Toast rice: Stir in Arborio rice and cook for about 2 minutes until edges are translucent.
Add broth gradually: Pour in warm vegetable broth one ladle at a time, allowing each addition to absorb before adding more (18-20 minutes total).
Finish with cheese: Once the rice is al dente, remove from heat and mix in butter and Parmesan until creamy.
Serve: Dish into bowls or plates, garnishing with fresh parsley if desired.
