Cook the onions. Melt the butter in a large, deep sauté pan over medium heat. Add the diced onion and season with the salt and black pepper. Cook for 4–5 minutes, stirring occasionally, until softened.

Brown the mushrooms. Add the sliced mushrooms to the pan and cook for 8–10 minutes, stirring occasionally, until they release their moisture, the liquid evaporates, and the mushrooms begin to brown. Adjust the heat as needed to encourage browning without burning the butter.

Add the garlic. Stir in the minced garlic and cook for about 30 seconds, until fragrant.

Make the roux. Sprinkle the flour over the mushroom mixture and stir until fully incorporated. Continue cooking for 1–2 minutes, stirring frequently, until no dry flour remains.

Make the creamy mushroom sauce. Gradually whisk in the chicken broth, adding a little at a time and stirring until smooth before adding more. Slowly stir in the half-and-half, Worcestershire sauce, and soy sauce.

Thicken the sauce. Bring the mixture to a gentle simmer over medium heat. Reduce the heat to medium-low and cook for 2–3 minutes, stirring frequently, until the sauce is thickened and creamy. Taste and adjust the salt and black pepper as needed.

Combine the casserole. Add the drained green beans and ½ cup of the French fried onions to the creamy mushroom sauce. Gently fold until everything is evenly coated. Reserve the remaining 1½ cups of crispy onions for topping.

Transfer to a baking dish. Spoon the green bean mixture into a lightly greased 8x8-inch or similar 2-quart baking dish. Proceed with the baking instructions below, or follow the make-ahead instructions if preparing in advance.

Preheat the oven to 350°F.

Transfer the prepared casserole to the oven and bake uncovered for 15–20 minutes, until hot and bubbling around the edges.

Remove from the oven and sprinkle the remaining 1½ cups of French fried onions evenly over the top.

Return to the oven and bake for another 10–15 minutes, until the onions are golden brown and crispy, the casserole is bubbling, and the center reaches 165°F.

Let rest for 5 minutes before serving.

Prepare the casserole through step 9, stirring ½ cup of crispy onions into the sauce and reserving the remaining 1½ cups for topping.

Allow the casserole to cool briefly, then cover tightly and refrigerate promptly, within 2 hours of cooking. Store in the refrigerator for up to 2 days before baking.

When ready to bake, preheat the oven to 350°F.

Remove any plastic wrap and cover the casserole with aluminum foil. Bake for 25–30 minutes, or until heated through.

Remove the foil and sprinkle the reserved 1½ cups of crispy onions evenly over the casserole.

Bake uncovered for another 10–15 minutes, until the topping is golden brown, the casserole is bubbling, and the center reaches 165°F.

Let rest for 5 minutes before serving.

Prepare the casserole through step 9, stirring ½ cup of crispy onions into the sauce. Transfer the mixture to a lightly greased, freezer-safe baking dish. Reserve the remaining 1½ cups of crispy onions for topping when baking.

Allow the casserole to cool briefly, then refrigerate until chilled, within 2 hours of cooking.

Wrap tightly with plastic wrap, followed by aluminum foil, and freeze for up to 2 weeks for best quality.

When ready to bake, transfer the frozen casserole to the refrigerator and allow it to thaw completely. Plan on at least 24 hours for thawing, although a deeper casserole may require longer.

Once completely thawed, preheat the oven to 350°F.

Remove the plastic wrap and cover the casserole with aluminum foil.

Bake covered for approximately 25–30 minutes, or until heated through. A casserole that is still very cold may require additional baking time.

Remove the foil and sprinkle the reserved 1½ cups of crispy onions evenly over the top.

Return to the oven and bake uncovered for another 10–15 minutes, until the crispy onions are golden brown, the casserole is bubbling, and the center reaches 165°F.

Let rest for 5 minutes before serving.