2. Pour the broth into a saucepan and bring to a simmer; adjust the heat under the broth to maintain a gentle simmer. Ladle out 1 cup of the hot broth, place in a bowl, add the saffron and set aside.
3. Melt the butter in a large sauté pan over medium heat. Add the onion and sauté until tender and translucent, about 5 to 8 minutes. Add the rice and stir until opaque, about 3 minutes. Add a ladleful of the simmering broth and stir until the broth is absorbed. Lower the heat and continue to add broth, a ladleful at a time.
4. For the final cup, add the saffron and its broth along with asparagus. Stir until the broth is absorbed and the rice kernels are al dente at the center and creamy on the outside, 20 to 25 minutes in all.
5. Combine the ingredients for the gremolata. When the rice is perfectly cooked, stir in the gremolata and heat through. Serve immediately. Pass the Parmesan if you like.