Heat olive oil in a pan and sauté sliced mushrooms until browned; set aside.
Melt 1 tbsp butter in a large skillet, sauté finely chopped onion and garlic paste until soft.
Add Arborio rice and toast for 2 minutes.
Gradually add warm vegetable broth, one ladle at a time, stirring constantly until absorbed before adding more.
Once rice is creamy and al dente (about 20 minutes), stir in cooked mushrooms, remaining butter, parmesan, black pepper, and salt.