1. Prepare the broth, if necessary. Bring the broth to a simmer over medium heat, then lower the heat so it is just keeping the broth hot. Remove ½ cup broth and put in a small bowl. Add the saffron and allow it to soak.

2. In a wide heavy saucepan, heat 2 tablespoons of the butter, the marrow if using, and the oil over medium heat. When the butter is melted, add the onion and cook, stirring often, until golden, about 10 minutes.

3. Add the rice and cook, stirring with a wooden spoon until hot, about 2 minutes. Add ½ cup of the hot broth and stir until the liquid is absorbed. Continue adding the broth ½ cup at a time, stirring after each addition. Adjust the heat so that the liquid simmers rapidly but the rice does not stick to the pan. About halfway through the cooking time, stir in the saffron mixture and salt and pepper to taste.

4. Use only as much of the broth as needed until the rice becomes tender yet firm to the bite. When you think it may be done, taste a few grains. If not ready, test again in a minute or so. If the broth runs out before the rice is tender, use hot water. Cooking time will be 18 to 20 minutes.

5. Remove the risotto pan from the heat and stir in the remaining 2 tablespoons of butter and the cheese until melted and creamy. Serve immediately.