Preheat your oven to 200°C (fan 180°C). While it heats, bring a large saucepan of salted water to a rolling boil. Drop in the green beans and cook for three minutes — they should turn a vibrant, bright green and feel just tender when you bite into one. Drain them immediately and run cold water over them to stop the cooking. Set them aside.

In a large ovenproof skillet or deep frying pan, warm the olive oil over a medium heat. Add the diced onion and cook for four to five minutes, stirring occasionally. The onion will become translucent and soft, and the kitchen will smell sweet and savoury. Add the garlic and cook for another minute until it becomes fragrant.

Tip the sliced mushrooms into the pan. Spread them in an even layer and let them sit undisturbed for two minutes — you want them to develop a golden-brown colour on one side, not steam. Then stir and cook for another three minutes. The mushrooms should look glossy, have reduced in volume noticeably, and release a deep, earthy aroma.

Sprinkle the flour over the vegetables. Stir constantly for one minute until the flour has completely disappeared into the mixture. The pan will look dry and pasty — this is exactly right. Slowly pour in the oat milk and vegetable stock, stirring all the while. The liquid will begin to thicken almost immediately.

Add the dried thyme, smoked paprika, salt, and pepper. Stir well and bring the sauce to a gentle simmer. You will see it bubble lazily around the edges, and the sauce will coat the back of a spoon. Let it cook for two minutes, then fold in the blanched green beans. Stir until every bean is coated in the creamy sauce.

Sprinkle the crispy fried onions evenly over the top of the casserole. Transfer the skillet to the preheated oven and bake for fifteen minutes. The top should turn a deep golden brown, and you will hear the sauce bubbling beneath the crust. If you want extra crunch, switch the oven to grill for the final two minutes, but watch it carefully so the onions do not burn.

Remove the casserole from the oven and let it rest for five minutes. This rest time allows the sauce to settle and thicken further. When you serve it, the beans will be tender, the sauce will be velvety, and the top will shatter with a satisfying crunch.