Remove stems from spinach and wash well under cold running water. Put wet spinach in a large saucepan with a pinch of salt, add 1 cup water and cook until spinach is tender. Drain in a colander, cool under running water and puree in a food processor.
Heat broth in a medium saucepan and keep warm over low heat.
Melt 3 tbsp. of the butter in a large skillet over medium heat. When butter foams, add onion and cook, stirring, until onion is lightly golden and soft, 5 to 6 minutes. Add rice and cook, stirring constantly until rice is translucent and well coated with the savory base, about 2 minutes. Add wine and stir until wine is almost all evaporated.
Now begin adding hot broth, 1 cup or so at a time or just enough to barely cover rice, stirring constantly until most of the broth has been absorbed. Continue cooking and stirring the rice in this manner, adding a small ladle of broth at a time for about 16 minutes.
Add pureed spinach. Cook and stir for 2 to 3 minutes, adding a little broth when needed. At this point, the rice should be tender but still a bit firm to the bite.
Stir in remaining butter and about 1/4 cup of the Parmigiano and stir for a minute or 2, until butter and cheese are melted and rice has a moist, creamy consistency. Taste, adjust seasoning and serve.