Place the dried porcini mushrooms in a bowl and add 2 cups of boiling water until just covered. Let the mushrooms hydrate fully for 20 to 30 minutes. Drain the mushrooms with sieve, making sure to keep and set aside the mushroom liquid for later.
Finely chop the hydrated mushrooms. Set aside. Place chicken broth and 1 cup of reserved mushroom broth in a separate saucepan. Bring to a low simmer.
Place a heavy bottom pot or Dutch oven over medium heat, add the butter and olive oil, and sauté finely diced onions, stirring constantly, until they become very soft and translucent.
With the heat over medium, add the Arborio rice and stir the mixture together. Toast the rice for 1 to 2 minutes, stirring constantly. Add a ladleful (roughly ½ cup (120mL)) of simmering stock, stirring constantly until the stock has been absorbed and the rice kernels appear almost dry. Adjust the heat as needed to prevent the liquid from evaporating too quickly.
Continue to add broth (using the same method) until the rice is al dente, soft on outside with a slight chewy interior. Stir in the chopped porcini, grated cheese, the remaining butter, and stir together. Season to taste with salt and pepper to taste. Serve immediately .