Bring the vegetable broth to a gentle simmer in a small pot, keeping it warm over low heat to have it ready for gradual addition.
In a wide heavy-bottomed pan, heat the olive oil over medium heat until it shimmers and you can smell the fragrant aroma, about 1-2 minutes.
Add the finely chopped onion to the pan and sauté slowly, stirring often, until it becomes translucent and soft, about 5 minutes. It should smell sweet and look glossy.
Stir in the rinsed Arborio rice and cook, stirring constantly, until the edges turn slightly translucent and it starts to smell toasted, about 2 minutes.
Begin adding the warm broth one ladleful at a time, stirring gently and waiting until the liquid is mostly absorbed before adding the next, maintaining a gentle simmer throughout.
Continue this process, stirring regularly and adding broth gradually, until the rice is tender but still slightly firm to the bite, about 18-20 minutes.
When the rice is nearly cooked, stir in the green peas and cook for another 2-3 minutes, just until they turn bright green and are tender.
Remove the pan from heat and stir in the grated Parmesan, butter (if using), and season with salt and pepper to taste. The risotto should be creamy and velvety, with the peas vibrant and fresh.
Let the risotto rest for a couple of minutes, then give it a gentle stir, tasting and adjusting seasoning if needed.
Spoon the risotto into bowls, garnish with extra Parmesan if desired, and serve immediately while hot and creamy.
