In a frying pan, heat up 1 tablespoon of olive oil over a medium heat. Add half of the mushrooms and sauté until cooked, about 5 minutes. Once the mushrooms are softened and gently colored, remove from the heat and set aside. Keep warm.
In a large, flat-bottomed pan, heat up the remaining olive oil with 1 tablespoon of butter. Add the finely chopped onion and gently sauté for 4 to 5 minutes. Add the other half of the mushrooms to the pan, along with the thyme, and then cook for another 4 to 5 minutes until everything is softened.
Add the arborio rice to the pan and stir gently for 1 to 2 minutes.
Pour in the white wine. Keep stirring and allow the rice to absorb the liquid.
When the wine is mostly absorbed, add a quarter of the chicken stock to the pan and stir it into the rice. Keep the pan to a simmer and stir frequently, feeding the risotto with more stock, bit by bit, when it has mostly absorbed the liquid already in the pan. After about 20 minutes the rice should start to become plump and creamy.
When all of the chicken stock has been fed into the risotto and it has become creamy, sample the rice to check that it is cooked enough. It should be softened but still have a slight bite to it.
Remove the risotto from the heat and add the remaining tablespoon of butter. Stir this in quickly to emulsify the rice, and then stir in the Parmesan.
Squeeze the roasted garlic bulb to remove the individual cloves. Lightly mash them and stir them into the risotto.
Season the risotto to taste and then serve hot with the reserved fried mushrooms, chopped basil, and Parmesan sprinkled on top if desired.