1. Preheat the oven to 400°F and position a rack in the middle. If your skillet isn’t oven-safe, prepare a 9×13-inch baking dish.

2. In a large oven-safe skillet or Dutch oven over medium-high heat, add the olive oil and 2 tablespoon of the butter. Once melted and foaming, add the chopped onion and sauté for 5-7 minutes until translucent. Then, add the minced garlic and cook for 1 more minute.

3. Add the sliced mushrooms to the skillet and stir to coat them with the aromatic mixture. Let them cook undisturbed for 4-5 minutes, then continue cooking (stirring occasionally) for another 10-12 minutes until their liquid has evaporated and the mushrooms are nicely browned and caramelized.

4. Push the mushrooms to the sides of the skillet, and add the remaining 2 tablespoon of butter to the center. Once melted, add the Arborio rice. Stir continuously for about 2 minutes until each grain is coated in butter and the edges appear slightly translucent.

5. If using wine, pour it in now and stir constantly, scraping up any browned bits from the bottom of the pan, until the wine has mostly evaporated.

6. Stir together the toasted rice and cooked mushrooms. Pour in the warmed vegetable broth, add the fresh thyme, chopped parsley, salt, and black pepper. Stir until well combined and bring the mixture to a gentle simmer on the stovetop.

7. Stir in half of the shredded Gruyère and half of the grated Parmesan. Immediately cover the skillet with a tight-fitting lid (or transfer the mixture to the prepared baking dish and cover with foil). Bake for 25-30 minutes until the rice has absorbed most of the liquid.

8. Remove the dish from the oven, remove the lid or foil, and sprinkle the remaining Gruyère and Parmesan evenly over the top.

9. Return the uncovered dish to the oven and bake an additional 10-15 minutes until the cheese is melted, bubbling, and the top is golden brown. Remove from the oven and let rest for at least 10 minutes. Garnish generously with fresh parsley before serving.