1. Preheat your oven to 375°F (190°C). Lightly grease a medium casserole dish to prevent sticking.
2. In a large skillet, melt the butter over medium heat. Add the chopped onion and sauté until translucent, about 5 minutes, stirring occasionally. This step builds the foundational sweetness that balances the mushrooms.
3. Add the minced garlic and sliced mushrooms to the pan. Cook until the mushrooms release their moisture and begin to brown, about 8-10 minutes. I like to stir frequently here to get even caramelization—this adds depth to the casserole.
4. Sprinkle the flour evenly over the mushroom mixture and stir well to coat. Cook for 1-2 minutes to remove the raw flour taste. This little trick helps thicken your sauce without lumps.
5. Gradually pour in the broth while stirring constantly to prevent clumping. Let the mixture simmer gently until it thickens, about 3-4 minutes.
6. Remove the skillet from heat and stir in the sour cream and half of the shredded cheese until smooth and creamy. Season with thyme, rosemary, parsley, salt, and pepper to taste. This is where the herbs really shine, lifting the earthiness of the mushrooms.
7. If using rice or noodles, gently fold them into the mushroom mixture now. This step is optional but makes the casserole heartier and perfect for a full meal.
8. Transfer the mixture to the prepared casserole dish. Sprinkle the remaining cheese evenly on top to create a golden, bubbly crust.
9. Bake uncovered for 20-25 minutes until the cheese is melted, bubbly, and lightly browned. Keep an eye on it—every oven is different, and I’ve learned that a little extra golden color makes all the difference.
10. Let the casserole rest for 5 minutes before serving. This helps everything set and makes it easier to dish out tidy portions.

Whenever I make this casserole, I think back to those early nights when I was just figuring out how to fix a dish gone wrong. This recipe’s step-by-step approach is designed to give you that same confidence I found after many trials and errors.