Warm about 1–1.5 liters of Beef Stock in a saucepan and, in a small ladle of the hot stock, crush a pinch of Saffron and let it steep to release its color and aroma; keep the rest of the stock just below a simmer.
Scoop the Beef Bone Marrow from the bones (or chop it finely) and, in a wide heavy skillet or Dutch oven, melt half of the Unsalted Butter over medium heat until the marrow dissolves into the butter.
Add one finely diced White Onion to the pan and sweat gently in the marrow-butter for 4–5 minutes until the onion is soft and translucent but not browned.
Stir in the Carnaroli Rice and toast it for 1–2 minutes, stirring constantly so each grain is coated and starts to become glossy.
Pour in the Dry White Wine and let it bubble away until mostly evaporated, stirring so nothing sticks to the pan.
Begin adding the hot stock a ladleful at a time (include some of the saffron-infused stock as you go), stirring frequently and allowing the rice to absorb most of the liquid before adding more; continue this process for about 16–18 minutes, until the rice is al dente and creamy.
Season gradually with Salt to taste during the cooking so the seasoning develops evenly.
When the rice is tender but still has a slight bite, remove the pan from the heat and vigorously stir in the remaining Unsalted Butter and finely grated Parmigiano Reggiano until the risotto is lush, glossy, and well combined; adjust consistency with a splash of hot stock if needed.
Cover the pan for a minute to let the risotto rest, then serve immediately while steaming hot, with an extra shave of Parmigiano if desired.
