Heat oil in a large, heavy-bottomed pan over medium heat.
Add chopped onions and mushrooms to the pan and cook until onions are soft and translucent, about 5-7 minutes.
Add arborio rice to the pan and stir until well combined with the onion and mushroom mixture.
In a separate pan, bring the chicken stock to a boil, then reduce to a gentle simmer.
Add 2/3 cup of hot stock to the rice mixture and stir continuously until all liquid is absorbed.
Continue adding stock in small amounts (about 1/2 cup at a time), stirring constantly and allowing each addition to be absorbed before adding more.
Repeat this process until all stock has been used and rice is creamy and tender, about 20-25 minutes.
Remove from heat and stir in the grated Parmesan cheese until melted and combined.
Serve immediately while hot.