Heat the oil over medium heat. Add the pancetta and cook, stirring occasionally, until golden, 4 to 6 minutes. Add the onion and continue to cook, stirring occasionally, until soft, 4 to 6 minutes longer.
Stir in the rice until well coated with fat. Add the vermouth and cook, stirring constantly until absorbed, about 1 minute. Add 1 cup of broth and cook, stirring constantly, until nearly absorbed, about 3 minutes. Continue adding the broth 1 cup at a time, stirring very frequently until each addition is absorbed, 20 to 25 minutes total. Add 1 cup of water and continue to cook, stirring frequently, until the rice is just barely done, 3 to 5 minutes more.
Add the scallops and spinach and cook, stirring constantly, until the scallops are just cooked, the spinach wilts, and the rice is mostly tender, with a little chew at the center, about 5 minutes longer. Stir in the Parmigiano and season to taste with salt. Serve immediately, sprinkling each portion with a little Parmigiano and a grind or two of pepper.