Begin by warming your broth in a saucepan over low heat. Keep it gently simmering throughout the cooking process.
Heat 1 tablespoon butter and 1 tablespoon olive oil in a large, heavy-bottomed pan over medium-high heat. Once hot, add the mushrooms in a single layer (work in batches if needed to avoid crowding). Season lightly with salt and pepper and cook undisturbed for 2-3 minutes until golden brown on one side.
Toss and continue cooking until mushrooms are caramelized and have released their moisture, about 5-7 minutes total. Transfer to a plate and set aside.
In the same pan, heat the remaining olive oil over medium heat. Add the diced onion and cook until translucent, about 3-4 minutes. Add the garlic and cook for an additional 30 seconds until fragrant.
Add the arborio rice to the pan and stir to coat with oil. Toast the rice for 2-3 minutes until the edges become slightly translucent while the centers remain white.
Pour in the white wine and stir constantly until almost completely absorbed, about 2 minutes.
Add one ladle (about ½ cup) of warm broth to the rice. Stir gently but frequently until the liquid is nearly absorbed. Continue adding broth one ladle at a time, allowing each addition to be absorbed before adding the next.
When you've used about two-thirds of the broth (approximately 15-20 minutes into the process), fold the sautéed mushrooms back into the risotto.
Continue adding broth and stirring until the rice is al dente – tender but with a slight firmness at the center. This typically takes 25-30 minutes total from when you first started adding broth.
Remove from heat and immediately stir in the remaining 2 tablespoons of butter, Parmesan cheese, and heavy cream (if using). Fold in the thyme leaves and adjust seasoning with salt and pepper to taste.
Garnish with fresh parsley and additional Parmesan if desired. Serve immediately.