Put the olive oil in a large, heavy saucepan over medium-low heat. Add the onion and pancetta and cook gently until the onion is translucent.
Add the fennel seed, and then the rice. Stir through so that the rice is coated in oil. Add the peas and some salt and pepper, then start adding the stock by the ladleful, stirring constantly and letting the liquid reduce a bit between additions. Do not let the rice become dry at any point -- it should be soupy at all times.
Cook for about 20 minutes, until the rice is al dente. Stir in the butter and Parmesan, along with a little more stock to loosen the rice if you'd like. Taste, adjust the seasonings and serve immediately, passing more grated Parmesan at the table.