Heat the olive oil in a large pan over medium heat.
Add the chopped onion and cook until translucent, about 5 minutes.
Add the minced garlic and sliced mushrooms, and cook until the mushrooms are tender, about 5 minutes.
Add the Arborio rice and stir to coat with the oil and vegetables.
Begin adding the vegetable broth, one ladle at a time, stirring frequently. Allow each addition of broth to be absorbed before adding the next.
Continue this process until the rice is creamy and cooked through, about 20 minutes.
Stir in the fresh spinach and cook until wilted, about 2 minutes.
Season with salt and pepper to taste, and serve hot.