Before you begin, reconstitute the dried porcini mushrooms. See "Recipe Notes" for instructions.
Bring broth to a very slow and steady simmer near the burner where you will be cooking the risotto.
In a large pot, add 1 tbsp butter, the oil, and the onions over medium heat. Cook until the onions turn translucent.
Add the rice and stir quickly to coat all of the grains.
Add ½ cup of simmering broth to the pot with the rice and stir constantly with a wooden spoon. Stir until all of the liquid is gone, taking care to wipe the sides and bottom clean with the spoon as you go so the rice does not stick.
When there is no more liquid, add another ½ cup of broth and repeat.
When the rice has cooked for 10 minutes, add the reconstituted mushrooms and half of their filtered water. Continue to stir and when there is no more water, add more of the mushroom water and repeat until it has all been used.
Finish cooking the rice with broth, or water if you run out of broth. Cook until the rice is tender, but firm, and no water remains in the pot. This is usually after 20 additional minutes of cook time.
Judge by taste, not just time. The risotto should be tender with a slight bite, so use the cooking time as a guide and adjust as needed for your batch.
Remove from the heat and add pepper, 1 tbsp butter, and Parmesan cheese. Stir to melt the cheese. Salt to taste.
Serve promptly with additional grated cheese on the side.
Notes
To reconstitute dried porcini mushrooms:
Soak the dried mushrooms (approx. 1 oz.) in 2 cups of barely warm water for at least 30 minutes.
Lift out the mushrooms by hand, squeezing back into the water to use in the recipe.
Rinse the mushrooms and remove any dirt. Pat dry with paper towels and chop or slice them for the recipe.
Filter the mushroom water through a strainer and use it in the recipe where it calls for "filtered mushroom water."