Melt the butter in a heavy large saucepan over medium heat. Add olive oil. Add the onions and saute until translucent and pink.
Next add garlic and sauté for 30 seconds or until the raw aroma of garlic disappears.
Now add both the chopped mushrooms to the pan and sauté until they are tender and juices evaporate, about 5 minutes.
In the meantime, measure Arborio rice in a bowl and rinse with water.
Add the washed rice to the pan and sauté for a few minutes. Next add a cup of water and cook until it's absorbed. Stir often. This would take around 2-3 minutes.
On a medium-high flame, add another cup of water and cook until it's absorbed. Stir often. This would again take around 2-3 minutes.
Continue to cook until the rice is just tender and the mixture is creamy, adding more water and stirring often, about 28 minutes (the rice will absorb 6 to 8 cups of water).
Water can be replaced by chicken broth or vegetable stock. This is optional.
Now stir in the thawed peas and parmesan. Season with salt and pepper.
Remove it to a serving/ storage dish. Your risotto is ready. Let it cool and refrigerate for a few hours or overnight, before using it to make "Arancini di Riso".