This is properly a side dish, meant to sit beside ossobuco, though we serve it on its own more often than not -- a full plate of saffron risotto has never once needed an excuse to exist by itself. Bone marrow is the traditional way to build the base, and we'll leave it here as an option rather than a requirement; the color and flavor of good saffron carry this dish a long way entirely on their own.
Serves 4 as a side, or 2–3 as a light main.
Steep the saffron threads in a small cup with a few tablespoons of the warm broth, and set it aside for at least fifteen minutes -- longer if you can manage it, even an hour. This is the step most home cooks rush, and it's the entire reason a good risotto alla Milanese tastes different from a merely yellow one.
Melt half the butter, and the bone marrow if using, in a wide, heavy pan over low heat. Add the onion, and cook gently until soft and translucent, without taking on any color, about five minutes.
Add the rice, and stir to coat it in the fat, toasting for two to three minutes, until the edges of the grains turn slightly translucent and it smells faintly nutty. Pour in the wine, and let it cook off entirely, stirring until you can no longer smell the alcohol.
Begin adding the warm broth, a ladle at a time, stirring often and waiting for most of each addition to be absorbed before adding the next. About ten minutes in, stir in the steeped saffron along with its soaking liquid. Continue this way for sixteen to eighteen minutes total, until the rice is tender but still has a slight bite at the center.
Remove from the heat. Beat in the remaining butter and the Parmigiano vigorously, off the heat, until the risotto turns glossy and loosens into something that moves like a slow wave when you tilt the pan -- all'onda, we call it. Season with salt, let it rest for a minute, and serve immediately.
If the risotto sits stiff and gluey on the plate instead of flowing, either too much liquid had already cooked away by the time you finished, or it rested too long before serving and seized up as it cooled; a risotto waits for no one, least of all the person still deciding what to drink with it. If the color turns out pale despite using real saffron, it wasn't given enough time to steep, or went in too early, before it had released its color into the broth. And if the texture turns gritty rather than smooth, cheap ground saffron was used in place of real threads, or the saffron was stirred in dry instead of properly steeped first.
A businessman, checking his watch roughly every ninety seconds, could not understand why a risotto took twenty full minutes to prepare when, in his words, it was "just rice," and asked, with what he clearly believed was a reasonable request, whether the kitchen might simply stir it faster. My father explained, with the patience of a man who has answered this exact question more times than he can count, that risotto cannot be rushed by effort, only ruined by the attempt -- the same way a phone call doesn't arrive any sooner for being dialed with urgency.
The businessman put his watch away, whether from understanding or simple resignation nobody could say for certain, and finished the entire plate without checking it again.