Prepare the Green Beans: In a large mixing bowl, blanch the trimmed green beans in boiling salted water for about 3 minutes. Drain and immediately transfer them to a bowl of ice water to stop the cooking process. Once cooled, drain and set aside.
Prepare the Sauce: To the skillet, add in the heavy cream, vegetable broth, soy sauce, Worcestershire sauce, Dijon mustard, salt, black pepper, and thyme. Stir well to combine, ensuring the mixture is uniform.
Combine Ingredients: Add the blanched green beans to the skillet, stirring until the beans are well-coated with the creamy sauce and evenly distributed.
Add Topping: Sprinkle half of the French fried onions over the green bean mixture. Stir gently to mix them in, while preserving some to remain crispy on top.
Bake the Casserole: Preheat your oven to 350. Transfer the skillet to the preheated oven and bake for about 25-30 minutes or until the casserole is bubbling and golden.
Finish with Crispy Onions: Remove the casserole from the oven and sprinkle the remaining French fried onions over the top. Return it to the oven for an additional 5 minutes to make the onions crispy.
Serve: Let the casserole cool for a few minutes before serving. Garnish with additional thyme if desired, and enjoy your delicious, creamy, one-pan green bean casserole!