Heat the broth in a medium saucepan; keep warm over low heat.

Melt 3 tablespoons of the butter in a large skillet over medium heat. When the butter begins to foam, add the arborio rice and stir for a minute or two, until the rice is well coated with the butter.

Add 2 ½ cups of the hot broth and bring to a boil. Reduce the heat to medium-low, partially cover, and simmer, stirring occasionally, until the rice has absorbed almost all of the broth and is tender but still firm to the bite, about 20 minutes. (If the broth absorbed too fast, add a bit more, as necessary.)

Prepare the sauce: heat the oil in a small skillet over medium heat. Add the onions and cook, stirring, until pale yellow and translucent, 6 to 7 minutes.

Add the sausage and pancetta and stir with a large wooden spoon, breaking up the sausage, until the sausage is lightly golden, 2 to 3 minutes.

Raise the heat to high and add the wine. Stir until the wine has evaporated.

Add the diluted tomato paste mixture, bring to a gentle simmer, and cook over medium-low heat, stirring occasionally, until the sauce has reduced by half and has a medium-thick consistency. Season lightly with salt and pepper.

Add the sauce to the cooked rice and cook and stir over medium heat until the sauce has been almost completely absorbed, 2 to 3 minutes.

Taste the rice, which should be tender but still a little firm to the bite. If it seems too firm, add a little more broth and cook a few minutes longer, until most of the liquid has been absorbed.

Add the remaining tablespoon of butter and the Parmigiano and stir until the butter is melted and the rice has a moist, creamy consistency.

Taste, adjust the seasoning, and serve immediately.