Blanch the green beans: Bring a large pot of salted water to a rolling boil. Add the beans and cook for exactly 3 minutes. They should turn a bright, jewel-like green and still offer a slight resistance when bitten. Drain immediately and plunge into a bowl of ice water to stop the cooking. Once cool, drain again and set aside.

Sauté the aromatics: In a large, deep frying pan, melt 2 tablespoons of butter over a medium heat. Add the sliced mushrooms and diced shallots. Cook, stirring occasionally, for 8–10 minutes. The mushrooms should release their liquid, then begin to brown, turning a deep golden colour and filling the kitchen with a nutty aroma. Stir in the minced garlic and cook for one minute more, until fragrant.

Build the sauce: Sprinkle the flour over the mushroom mixture and stir constantly for 2 minutes. The paste should smell toasty and look slightly clumpy. Slowly pour in the milk, cream, and stock, whisking continuously to prevent lumps. Bring the sauce to a gentle simmer – you’ll see small bubbles breaking the surface. Cook for 4–5 minutes, until the sauce thickens enough to coat the back of a spoon. Season with salt, pepper, and fresh thyme.

Combine and transfer: Add the drained green beans to the pan and stir well, ensuring every bean is coated in the creamy sauce. The mixture should look glossy and inviting. Pour everything into a 9x13 inch baking dish, spreading it into an even layer.

Make the topping: In a small bowl, combine the fresh breadcrumbs, grated Parmesan, and olive oil. Toss with your fingers until the crumbs are evenly moistened. Sprinkle the mixture generously over the casserole.

Bake to golden perfection: Preheat your oven to 180°C (350°F). Place the dish on the middle rack and bake for 25–30 minutes. The casserole is ready when the sauce is bubbling around the edges and the topping is a deep, crispy golden brown. The smell should be intoxicating – buttery, savoury, and slightly nutty from the cheese.

Rest before serving: Remove from the oven and let the casserole rest for 10 minutes. This allows the sauce to settle and thicken slightly, so you get clean, neat servings. The first spoonful should reveal tender beans swathed in a rich, velvety sauce, capped with a satisfying crunch.