Follow these detailed steps to ensure a perfectly creamy and flavorful garlic mushroom rice bake every time. The process is straightforward and primarily uses one oven-safe skillet or Dutch oven.

1. Preheat and Prepare: Begin by preheating your oven to 400°F (200°C). Position a rack in the middle of the oven. This ensures the bake cooks evenly from top to bottom. If your skillet is not oven-safe, have a 9×13 inch (or similar 3-quart) baking dish ready.
2. Sauté the Aromatics: Place a large, oven-safe skillet or Dutch oven over medium-high heat. Add the 2 tablespoons of olive oil and 2 tablespoons of the butter. Once the butter is melted and foaming, add the finely chopped yellow onion. Sauté for 5-7 minutes, stirring occasionally, until the onion has softened and become translucent. Add the minced garlic and cook for another minute until fragrant. Be careful not to let the garlic burn, as it can become bitter.
3. Cook the Mushrooms: Add the sliced Cremini mushrooms to the skillet. It might seem like a lot of mushrooms, but they will cook down significantly. Stir to coat them in the onion and garlic mixture. Let the mushrooms cook undisturbed for 4-5 minutes. This allows them to release their liquid and begin to brown, a process which develops their deep, savory flavor. Continue to cook, stirring occasionally, for another 10-12 minutes, or until the liquid has evaporated and the mushrooms are beautifully browned and caramelized.
4. Toast the Rice: Push the mushrooms to the sides of the skillet and add the remaining 2 tablespoons of butter to the center. Once melted, pour in the uncooked Arborio rice. Stir continuously for about 2 minutes, ensuring every grain of rice is coated in the butter and becomes slightly translucent at the edges. This toasting step is crucial; it gives the rice a nutty flavor and helps the grains maintain their individual structure in the final dish.
5. Deglaze the Pan: If using, pour in the dry white wine. Stir constantly, scraping up any browned bits from the bottom of the pan with your wooden spoon. This process, called deglazing, incorporates all those flavorful bits back into the dish. Continue cooking until the wine has almost completely evaporated.
6. Combine and Simmer: Stir the toasted rice and the cooked mushrooms together. Pour in the warmed vegetable broth, fresh thyme, chopped parsley, salt, and freshly ground black pepper. Stir everything well to combine. Bring the mixture to a gentle simmer on the stovetop.
7. First Bake: Once simmering, stir in half of the shredded Gruyère cheese and half of the grated Parmesan cheese. This introduces creaminess deep within the rice bake. Immediately cover the skillet with a tight-fitting, oven-safe lid (or transfer the mixture to your prepared baking dish and cover tightly with foil). Place the dish in the preheated oven.
8. Bake to Perfection: Bake for 25-30 minutes. By this time, the rice should have absorbed most of the liquid. Carefully remove the dish from the oven. Remove the lid or foil. The rice should look mostly cooked but may have a little liquid remaining on the surface.
9. Create the Cheesy Crust: Sprinkle the remaining Gruyère and Parmesan cheese evenly over the top of the rice bake.
10. Final Bake and Rest: Return the dish to the oven, uncovered. Bake for an additional 10-15 minutes, or until the cheese is completely melted, bubbling, and has formed a beautiful golden-brown crust on top.
11. Rest and Garnish: Remove the bake from the oven and let it rest for at least 10 minutes before serving. This crucial resting period allows the rice to absorb any final bits of liquid and helps the dish set, making it creamier and easier to serve. Garnish generously with fresh parsley before bringing it to the table.