Preheat the oven to 180℃.
Melt the butter in a large frying pan over medium heat. Add the oil and diced onions. Cook whilst stirring for 5 minutes or until the onions are soft and translucent.
Add the mushrooms and thyme to the pan. Cook for 7-8 minutes or until the mushrooms are tender and juice has evaporated from the pan. Season with salt and pepper.
Add the cream to the pan. Stir to combine then reduce heat to low and simmer gently for 10 minutes or until the sauce has thickened slightly.
In the meantime, combine bread crumbs in a small bowl with the melted butter and chopped parsley. Set aside.
Once the sauce has thickened, transfer the mushrooms to an ovenproof dish and top with the breadcrumb mixture.
Bake the gratin for 10-15 minutes. Remove from the oven and allow to rest for 5 minutes before serving.
Serve the gratin with chicken or fish.
4 servings