Heat stock and saffron in a 2-qt. saucepan over medium heat; keep warm.
Heat butter in a 4-qt. saucepan over medium-high heat.
Add onions; cook until soft, about 4 minutes.
Add rice; cook until lightly toasted, about 4 minutes.
Add wine; cook until evaporated, about 2 minutes.
Add ½ cup warm stock; cook, stirring, until absorbed, about 2 minutes.
Continue adding stock, ½ cup at a time, and cooking until absorbed before adding more, until rice is tender and creamy, about 16 minutes total.
Stir in Parmesan; season with salt and pepper.