Blanch the Green Beans: Bring a large pot of salted water to a rolling boil. Add the green beans and cook for 3-4 minutes. You will see them turn a bright, vivid emerald green and they will become slightly tender but still have a firm snap when bitten. Immediately drain and plunge them into a bowl of ice water to stop the cooking process. Once cool, drain them thoroughly and pat dry with a clean tea towel. This step locks in the colour and crunch.

Prepare the Crispy Onions: While the beans cool, preheat your oven to 180°C (fan 160°C). Spread the crispy fried onions on a baking tray lined with baking paper. Bake for 5-7 minutes, until they are deeply golden and smell wonderfully toasty. They should be a shade darker than when they went in. Set aside to cool; they will become even crisper as they do.

Sauté the Mushrooms: In a large, deep frying pan or saucepan, melt the butter over a medium-high heat. Add the chopped mushrooms and a pinch of salt. Sauté for 6-8 minutes, stirring occasionally. You will hear a gentle sizzle, and the mushrooms will release their liquid, which will then evaporate. The mushrooms will shrink and turn a lovely golden brown, with a rich, earthy aroma.

Build the Sauce: Add the minced garlic to the pan and cook for 30 seconds until fragrant. Sprinkle over the plain flour and stir continuously for 1 minute. The mixture will look like a thick, golden paste (a roux) and will smell nutty. Slowly pour in the milk and stock, whisking constantly to prevent lumps. Bring the sauce to a gentle simmer. You will see it begin to thicken, coating the back of a spoon. This should take about 3-4 minutes.

Finish the Sauce: Stir in the fresh thyme leaves, black pepper, and a pinch of freshly grated nutmeg. Taste and adjust the seasoning, remembering the Parmesan will add more saltiness. Let the sauce bubble gently for another 2 minutes. The texture should be glossy, smooth, and thick enough to coat the green beans without being gluggy.

Combine and Assemble: Add the blanched, dry green beans to the pan of sauce. Fold them in gently with a spatula until every bean is evenly coated in the creamy, savoury sauce. You will see the bright green beans beautifully enrobed in the pale, creamy sauce. Transfer the mixture to a 9x13 inch baking dish or into individual gratin dishes for a more elegant presentation.

Bake and Top: Bake the casserole (uncovered) in the preheated oven for 15 minutes. The sauce should be bubbling softly around the edges and the top should be just starting to set. Remove from the oven and sprinkle the reserved crispy fried onions and the grated Parmesan cheese evenly over the top. Return to the oven and bake for a final 10-12 minutes. The cheese will melt and the onions will become an irresistible, crunchy golden crust. The entire kitchen will smell of rich, savoury comfort.

Rest and Serve: Let the casserole rest for 5 minutes before serving. This allows the sauce to set slightly, making it easier to serve neat, elegant portions. Garnish with a few fresh thyme sprigs for a final restaurant-quality touch.