Bring a pot of water to a boil, then add the green beans and blanch for 3 minutes until vibrant green and slightly tender. Drain and set aside.
Heat butter in a large sauté pan over medium heat until it melts and starts to sizzle. Add sliced onion and cook for about 8 minutes, stirring often, until golden and fragrant. The onions should smell sweet and caramelized.
Stir in the cream of mushroom soup and warm through for about 2 minutes until it starts bubbling and smells earthy and savory. If it’s too thick, add a splash of milk or broth to loosen.
Combine the blanched green beans with the mushroom sauce in a mixing bowl, gently tossing to coat evenly. Season with salt and pepper to taste.
Pour the mixture into your prepared baking dish, spreading it evenly with a spatula. Sprinkle the toasted fried onions on top for that crispy, golden finish.
Bake uncovered in a preheated oven at 180°C (350°F) for 20-25 minutes until bubbling around the edges and the top is golden and crispy. The aroma should be warm and inviting.
Remove from the oven and let it rest for 5 minutes to allow the casserole to set. This makes serving easier and ensures the topping stays crisp.
Serve hot, with extra fried onions if desired, and enjoy the delightful contrast between the creamy beans and crunchy topping.
