In a large pot, bring stock to a gentle simmer.
Poach marrow pieces in simmering stock (2-3 minutes), remove and reserve.
In another large pot, heat the olive oil and butter and cook onions until softened.
Add arborio rice, stir to coat.
Add wine, stir to combine and evaporate.
Reduce heat to medium-low. Add saffron infusion (as per packet instructions), and a ladle of the simmering stock. Cook, stirring constantly, until most of the liquid is absorbed. Continue adding stock, a ladle at a time as the liquid is absorbed, stirring constantly. This takes about 20-25 minutes.
When the last of the stock is used and the rice is almost tender, add marrow pieces, pecorino cheese and parsley. Stir briefly to combine and season lightly
Serve in large bowls, garnished with parmesan cheese