Heat the olive oil in a heavy-bottomed pan over medium heat until it shimmers and begins to smell fragrant. Add the finely chopped onion and cook, stirring occasionally, until translucent and soft, about 3-4 minutes.
Add the Arborio rice to the pan and stir to coat each grain with oil. Toast the rice gently, stirring constantly, until the edges turn slightly translucent and it emits a nutty aroma, about 2 minutes.
Pour in the white wine (if using) and stir until it is mostly absorbed, about 1-2 minutes. The rice will start to look glossy and plump.
Begin adding the warm vegetable broth, one ladleful (about 1/2 cup) at a time. Stir constantly, allowing each addition to be absorbed before adding the next. Keep the heat at a gentle simmer to prevent scorching.
Continue adding broth and stirring, tasting the rice after about 15 minutes. When the rice is just tender but still has a slight bite, and the mixture is creamy, add the green peas and stir to combine. Cook for an additional 3-4 minutes until peas are bright green and tender.
Remove the pan from heat and stir in the butter and freshly grated Parmesan cheese until melted and silky. Add lemon zest if desired, and season with salt and freshly ground pepper to taste.
Let the risotto rest for a minute or two, then give it a gentle stir. The final dish should be creamy, glossy, and vibrant with the green peas visible throughout. Serve immediately, garnished with extra Parmesan or herbs if you like.
