Making this mushroom casserole is straightforward and fun, with steps that flow smoothly from start to finish. It begins with preheating the oven and ends with a hot, bubbly dish ready to serve. The total time is about 40 minutes, so it’s perfect for a quick dinner.

First, preheat the oven to 400°F and grease a 2-quart baking dish with olive oil. This sets the stage for even cooking and prevents sticking. Next, heat olive oil in a large deep skillet over medium-high heat, then add the mushrooms, onions, and kosher salt.

Cook the mixture while stirring occasionally until the mushrooms release most of their liquid and it evaporates, which takes about 10 minutes. This step helps build flavor and avoid a watery casserole. Add the garlic, black pepper, and thyme, and cook for one more minute before removing from heat.

Transfer the cooked mixture to the prepared baking dish and spread it evenly.

Sprinkle the shredded Gruyere cheese on top for a gooey finish.

Bake for approximately 15 minutes until hot and bubbly, then garnish with dried parsley before serving.

Once baked, let it cool slightly to make serving easier and enjoy the rich flavors.