Let’s bring this comforting dish to life with these easy steps:

1. Preheat your oven to 375°F (190°C).
2. In a large skillet, heat olive oil over medium heat. Add the sliced mushrooms and cook until they are golden brown, about 5-7 minutes.
3. Add the chopped onion and garlic to the skillet, sautéing for another 3 minutes until the onions are translucent.
4. Sprinkle the flour over the mushroom mixture, stirring well to coat evenly.
5. Gradually pour in the chicken or vegetable broth and heavy cream, stirring constantly until the sauce thickens, about 5 minutes.
6. Stir in the dried thyme, rosemary, salt, and pepper. Let the mixture simmer for another 2 minutes.
7. Transfer the creamy mixture to a baking dish. Sprinkle Parmesan cheese over the top.
8. In a small bowl, combine breadcrumbs and melted butter. Spread this mixture evenly over the casserole.
9. Bake in the preheated oven for 20-25 minutes, or until the top is golden brown and bubbly.
10. Garnish with fresh parsley before serving.

The secret to this dish is allowing the mushrooms to caramelize, bringing out their natural sweetness and depth of flavor. It’s a little trick Mum taught me, and it never fails to impress.