In a large pot, heat the olive oil over medium heat.
Add the diced onion and minced garlic, and cook until soft, for about 3-4 minutes.
Add the diced Lion’s Mane mushrooms to the pot and cook for 5-7 minutes until they release their moisture and are lightly browned.
Add the Arborio rice to the pot and stir well to coat with the onion and mushroom mixture.
Gradually add the vegetable broth, 1/2 cup at a time, stirring frequently until the liquid is absorbed before adding more. Continue cooking and stirring until the rice is tender and creamy, for about 25-30 minutes.
Remove the pot from heat, and stir in the butter and Parmesan cheese until well combined. Season with salt and pepper to taste.
Serve hot, garnished with additional grated Parmesan cheese and chopped fresh parsley, if desired.