Pour about 1/2 cup of the hot beef stock into a small heatproof bowl and add the saffron threads. Let them sit and steep so the strands soften and release their color and aroma while you begin the risotto.

In a wide, heavy 3- to 4-quart casserole or pot, warm the olive oil over medium heat. Add the minced onion and shallots and cook, stirring frequently, until they turn a rich golden color and become soft and fragrant, about 8 minutes. Tip in the rice and stir thoroughly so every grain is coated in the oil and aromatics. Toast the rice gently for 1 to 2 minutes, until the edges of the grains look slightly translucent while the centers remain opaque.

Pour in the white wine and stir steadily until it has almost completely evaporated and you no longer smell raw alcohol. Add 1/2 cup of the remaining hot stock along with 1/2 teaspoon of salt. Cook the rice, stirring constantly, until the liquid is fully absorbed and the mixture thickens slightly. If using the beef marrow, stir it in at this stage so it melts into the rice and enriches the base.

Continue cooking the risotto by adding hot stock in small increments, just enough each time to moisten the rice. Stir continuously and wait for each addition to be absorbed before adding more stock. About 10 minutes after you begin adding the stock, pour in the saffron and its soaking liquid, stirring well so the color and flavor distribute evenly.

Maintain a gentle simmer, adjusting the heat as needed so the rice bubbles softly rather than boiling vigorously. Keep adding stock and stirring until the risotto becomes creamy and the grains are al dente—tender with a slight bite at the center. This stage usually takes 16 to 20 minutes from the moment the wine was added.

Remove the pot from the heat. Immediately beat in the butter bits until they are fully melted and the risotto turns glossy and silky. Stir in the grated Parmigiano-Reggiano, mixing until the cheese is completely incorporated and the texture is smooth and luxurious. Taste and adjust with a little more salt if needed, and finish with freshly ground black or white pepper.

Serve the risotto at once, ladling it into warm shallow bowls so it holds its creamy texture and vivid saffron color.