Bring a pot of water to a boil. Prepare a large bowl as an ice bath.
Add the trimmed green beans to the boiling water for 2-3 minutes (or until the water starts boiling again).
Remove the green beans and immediately transfer them to the ice bath to halt the cooking process.
Preheat the oven to 375°F (190°C). In a large skillet over medium-high heat, melt the butter and add the sliced mushrooms.
Saute the mushrooms for 4-5 minutes, or until they have started to lose their moisture.
Add the paprika, cayenne pepper, spicy brown mustard, and minced garlic to the skillet with the mushrooms.
Coat the mushrooms in the spices and let them simmer for 2-3 minutes, allowing the garlic to become fragrant. Season with salt and pepper.
Sprinkle the flour into the pan, ensuring the mushrooms are completely covered.
Pour in the chicken broth and bring the sauce to a simmer for 2-3 minutes.
Reduce the heat to low and add the sour cream and heavy whipping cream.
Stir until the creams are fully incorporated. Let the sauce stand and thicken for 3-4 minutes.
In a casserole pan, arrange the blanched green beans on the bottom, filling any open spaces.
Pour the mushroom sauce evenly over the green beans, creating a luscious layer.
Bake the casserole for 25-30 minutes or until the top is bubbling and just beginning to turn golden brown.
For an added touch, sprinkle fried onions or mozzarella cheese on top, if desired. Serve and enjoy!