In a large saucepan over medium, combine the chopped onion, pancetta and 2 tablespoons butter.
Cook, stirring occasionally, until the onion is lightly browned and the pancetta is rendered and lightly browned, 6 to 8 minutes.
Add the rice and stir until the grains are coated with fat, then stir in 1 cup of the broth.
Cook, stirring, until the liquid is mostly absorbed, about 5 minutes.
Ladle in additional broth to barely cover the rice and simmer, stirring often, until the broth is mostly absorbed.
Repeat the addition of broth and simmering until mostly absorbed 4 or 5 times, until the rice is al dente and most of the broth has been used; this process should take 25 to 30 minutes.