In a small bowl, dissolve the saffron in 1/2 cup of broth and set aside. Set a heavy nonreactive saucepan over medium-high heat. Add enough butter to sauté the cut onions until tender, about 3 minutes. Add the rice, stir well to coat the grains, then deglaze with the white wine. Stir well. Add the saffron broth, and then add the hot broth in stages. As it is absorbed, add more broth to cover. When rice is tender and broth is absorbed (about 25 minutes), add the cooked peas and finish risotto with remaining whole butter and fresh-grated cheese.