Ingredients:
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 1⁄2 cup warm stock; cook, stirring, until absorbed, about 2 minutes. Continue adding stock, 1⁄2 cup at a time, and cooking until absorbed before adding more, until rice is tender and creamy, about 16 minutes total. Stir in marrow, if using, and Parmesan; season with salt and pepper.