Step 1: Prepare Your Green Beans. If using fresh green beans, trim the ends. Bring a large pot of salted water to a rolling boil. Add the trimmed green beans and blanch for 3-5 minutes, until bright green and crisp-tender. Immediately transfer them to an ice bath to stop the cooking process. Drain thoroughly and pat dry. If using frozen green beans, thaw them completely and pat dry with paper towels to remove excess moisture.
Step 2: Sauté the Mushrooms and Onion. In a large, oven-safe skillet or Dutch oven, melt 2 tablespoons of butter over medium heat. Add the sliced mushrooms and chopped onion and cook, stirring occasionally, until they release their liquid and start to brown, about 7-10 minutes. Add minced garlic and cook for another minute until fragrant. Remove the mushroom mixture from the skillet and set aside.
Step 3: Make the Cream Sauce. In the same skillet, add the remaining 3 tablespoons of butter. Once melted, sprinkle in the flour and whisk continuously for 1-2 minutes to create a roux. Gradually pour in the milk and heavy cream, whisking constantly to prevent lumps. Bring the mixture to a simmer, then reduce heat to low and cook, stirring, until the sauce thickens to your desired consistency (about 5-7 minutes). It should be thick enough to coat the back of a spoon.
Step 4: Season the Sauce. Stir in the Worcestershire sauce, Dijon mustard, salt, and black pepper. Taste and adjust seasonings as needed. This is your chance to make the sauce perfectly flavorful for your green bean casserole recipe.
Step 5: Combine Ingredients. Add the blanched green beans and sautéed mushroom-onion mixture back into the skillet with the cream sauce. Stir gently to ensure everything is evenly coated. Fold in about half of the crispy fried onions, reserving the rest for topping.
Step 6: Transfer to Baking Dish (if not using an oven-safe skillet). If your skillet is not oven-safe, transfer the green bean mixture to a 9x13-inch baking dish.
Step 7: Bake the Casserole. Preheat your oven to 375°F (190°C). Bake the casserole, uncovered, for 20 minutes, or until bubbly and heated through.
Step 8: Add Final Topping. Remove the casserole from the oven. Sprinkle the remaining crispy fried onions evenly over the top. Return to the oven and bake for another 5-10 minutes, or until the fried onions are golden brown and extra crispy. Keep a close eye on them to prevent burning.
Step 9: Serve. Let the green bean casserole recipe rest for a few minutes before serving. This allows the sauce to set slightly. Serve hot and enjoy this delicious green bean casserole recipe!