In a saucepan, heat the broth and keep it warm over low heat.
In a large skillet, heat olive oil and butter over medium heat.
Add the chopped onion and sauté until translucent.
Stir in the garlic and cook for another minute.
Add the sliced mushrooms and sauté until they are tender and browned.
Add the Arborio rice to the skillet and stir to coat the rice with the oil.
Pour in the white wine, stirring until it is absorbed.
Gradually add the warm broth, one ladle at a time, stirring constantly.
Continue adding broth as it's absorbed, for about 18-20 minutes, until the rice is creamy and al dente.
Stir in the Parmesan cheese, season with salt and pepper, and serve hot.