Pour the beef broth into a saucepan and heat. Once hot, remove 1/2 cup of broth and place in a bowl with the saffron threads. Keep the remaining broth warm over medium low heat.

Pour 1 ½ cups hot water over the dried porcini mushrooms and let soak until softened, about 20 minutes. Then, strain the mushrooms, reserving the soaking liquid. You should have about 1 cup of mushroom broth. Chop the softened mushrooms to desired size and set aside.

In a large pan, melt 2 tablespoons butter in the olive oil. Add in the chopped onions and cook until softened, approximately 5 minutes.

Next, add in the rice and stir, making sure to coat with the butter, oil, and mushrooms. Pour in the wine and cook, stirring continuously until the wine evaporates.

Once the wine evaporates, pour in the saffron broth and cook, until the rice absorbs the broth. Next add the mushroom broth 1/2 cup at a time, stirring after each addition, waiting for the rice to absorb the broth before adding more.

Continue cooking the rice in this manner, adding ladlefuls of the hot broth, stirring until absorbed, before adding more. Do this until the rice is tender, about 20 minutes. Place in the chopped mushrooms and stir to combine.

Take the pan off the heat, sprinkle on the Parmigiano Reggiano cheese as well as the remaining 2 tablespoons of butter and stir to combine before serving.