Heat olive oil in a large, heavy-bottomed pot or Dutch oven over medium heat. Add onion and cook until softened and translucent, about 5-7 minutes. Add garlic and cook for another minute until fragrant.

Add sliced mushrooms and cook until they release their liquid and start to brown, about 5-8 minutes. Season lightly with salt and pepper. Remove about a quarter of the cooked mushrooms and set aside for garnish if desired.

Add Arborio rice to the pot. Stir constantly for 2-3 minutes, toasting the grains until they become slightly translucent at the edges but remain opaque in the center.

Pour in the white wine, stirring until it is completely absorbed by the rice.

Begin adding the hot vegetable broth, one ladleful (about half a cup) at a time, stirring frequently. Wait until each ladleful is almost fully absorbed before adding the next. This process should take about 20-25 minutes.

Continue adding broth and stirring until the rice is al dente, meaning it's tender but still has a slight bite in the center. The risotto should be creamy and flow slightly.

Remove the pot from the heat. Stir in the cold, diced butter and half of the grated Parmesan cheese. Stir vigorously (mantecatura) for 1-2 minutes until the butter and cheese are fully incorporated and the risotto is incredibly creamy.

Taste and adjust seasoning with salt and pepper as needed.

Serve immediately in warm bowls, garnished with the remaining Parmesan cheese, fresh chopped parsley, and the reserved mushrooms if using.