Bring a large pot of salted water to a boil, then add the green beans and blanch for 3-4 minutes until they turn bright green and just tender. Drain well and set aside.
While the beans cook, melt a tablespoon of butter in a skillet over medium heat. Add the sliced mushrooms and sauté for about 8-10 minutes, stirring occasionally, until they become golden and smoky smelling.
In a large mixing bowl, combine the blanched green beans with the cream of mushroom soup and the sautéed mushrooms. Season with smoked paprika and freshly ground black pepper, then stir until everything is evenly coated and well mixed.
Transfer the mixture into your prepared baking dish, spreading it out evenly. Top generously with crispy onions, making sure they’re distributed across the surface for maximum crunch.
Bake uncovered in a preheated oven at 175°C (350°F) for 20-25 minutes, until the mixture is bubbling around the edges and the top is golden brown. If the onions start browning too quickly, loosely cover the dish with foil to prevent burning.
Once baked, remove the casserole from the oven and let it rest for 5 minutes. This helps the sauce thicken slightly and makes serving easier. For an extra savory touch, sprinkle crumbled prosciutto or additional crispy onions on top before serving.
