Sauté sliced mushrooms in olive oil until browned; set aside.
In a baking dish, combine arborio rice, chopped onion, garlic paste, hot vegetable broth, butter, pepper, and salt.
Cover tightly with foil and bake at 190°C for 35 minutes until rice is tender and liquid is absorbed.
Remove foil, stir in cooked mushrooms and grated parmesan cheese vigorously for 2 minutes to make it creamy.