In a small bowl, add the optional porcini mushrooms and cover with 1 1/2 cups (350ml) of hot water. Set aside for 20-30 minutes until the mushrooms are soft. With your finger tips, carefully pick the soft mushrooms out of the soaking liquid and gently squeeze out excess water. Roughly chop the mushrooms.
If using porcini, pour the soaking liquid into a small saucepan along with the chicken broth. Take great care not to get any of the grit in the bottom of the bowl into the pot. (If you want to be very sure, pour the soaking liquid through a coffee filter.) You should have about 4 cups of liquid total. Bring the broth to a boil over medium high heat. Reduce the heat to very low and keep the broth at a bare simmer as you add it to the rice later on. Check the broth for seasoning and add a little salt if necessary.
Meanwhile, in a large skillet, heat 1 TBS of oil and 1 TBS butter over medium high heat. Add the mushrooms along with a good pinch of salt. Sauté 6-8 minutes, stirring occasionally, until the mushrooms are cooked. Add the thyme and a few grinds of pepper, stir to combine and set aside.
In a 3-4 quart (3-4L) pot, heat the remaining 3 TBS oil over medium high heat until shimmering. Add the chopped onion and porcini mushrooms and cook 4-5 minutes until the onion is soft and translucent. The onion should not have any color on it. Add the minced garlic and cook 30-60 seconds until the garlic becomes very fragrant. Take care not to overcook or burn the garlic. Add the rice and stir to coat the grains with oil. Continue stirring about 2 minutes to "toast" the rice. That does not mean the rice will change color; you will know when it is ready to receive the broth when the outer edges of the rice grains become translucent, the rice will feel heavy as you stir it in the pot and the grains will make a tinkling sound as they hit the sides of the pot.
Add the wine and stir constantly until the alcohol evaporates and the pot is almost dry. Add about half a cup of the barely simmering broth and stir until it has been absorbed by the rice and the pot is almost dry. Add another half cup of broth and repeat this process until the rice is cooked. The process should take about 18 minutes from the time you begin adding the broth. At the 15 minute mark taste a few grains of rice to see how far it needs to go. It will be quite definitely undercooked but it will let you know how close you are.
At this point add about 1/4 cup of broth to the mushrooms and rewarm over medium heat, scraping up any browned bits on the bottom of the pan. Add the mushrooms to the rice along with the next addition of broth. Continue cooking another 2-3 minutes, adding broth as needed until the rice is cooked but still very slightly al dente. Stir in a final scoop of broth and immediately remove the pot from the heat, cover and set aside 5 minutes.
Stir in the cheese and the remaining tablespoon of butter. For a special flavor boost, stir in about a teaspoon of truffle oil. Serve the risotto immediately in warm bowls.