Bring broth to a simmer in a small pot. Reduce heat to the lowest possible setting.
Melt butter in a heavy-bottomed pot or Dutch oven. Add onion and a big pinch of salt. Saute over medium-low heat until onion is soft and translucent but not brown, about 10 minutes. Add garlic and saute until fragrant, about 1 minute.
Increase heat to medium. Add rice and saffron and stir well. Cook, stirring every minute or two, until edges of rice are transparent, about 4 minutes.
Add wine and stir well. Simmer until liquid is absorbed and bottom of pot is mostly dry, about 5 minutes.
Add 1 1/2 cups hot broth and stir well. Simmer, stirring every 3-4 minutes, until liquid is absorbed and bottom of pot is mostly dry, 10-12 minutes.
Add more hot broth, about 1/2 cup at a time. Stir after each addition and cook until liquid is absorbed, about 3-4 minutes. It should take 3-4 additions of broth to get the rice cooked; it should still have just a hint of al dente bite in the center of each grain. (The rice can be held in a 350F oven for about 30 minutes after the third addition of broth if necessary.).
Season to taste with salt and white pepper; stir in Parmigiano and serve at once in warmed bowls.