Heat the stock and dissolve the saffron in a small amount of it. Keep warm.
In a large pan, melt half of the butter and sauté the onion until soft.
Add the rice and toast for 2 minutes, stirring constantly.
Pour in the white wine and let it evaporate.
Start adding the hot stock, one ladle at a time, stirring until absorbed before adding more. Continue for about 18–20 minutes.
When the rice is al dente, remove from heat. Stir in the remaining butter and grated Parmigiano. Season with salt and pepper.
Serve immediately, creamy and hot.