Preheat your oven to 350°F (175°C). In a large skillet over medium heat, melt the butter. Add the finely diced onion and sliced mushrooms, sautéing until just tender and the mushrooms have released some of their moisture, about 5 to 7 minutes. Stir occasionally to prevent sticking. Add the minced garlic and cook for another 1 to 2 minutes until fragrant. Remove the skillet from the heat and set the mixture aside.

In a medium saucepan, combine the trimmed and halved green beans with the low-sodium chicken broth. Bring the mixture to a boil over medium-high heat and continue cooking for about 8 minutes, or until the green beans are crisp-tender. Drain the green beans thoroughly and set them aside. I always recommend checking the beans for doneness a minute early—they should still have a slight snap for the best texture in the final casserole.

Spray a 2-quart baking dish with cooking spray or lightly grease it. In a large bowl, combine the cream of mushroom soup, sautéed mushroom mixture from Step 1, cooked green beans from Step 2, grated Parmesan cheese, salt, and ground black pepper. Stir everything together until it is well mixed. The soup and cheese will help bind the casserole and add creaminess and flavor.

Transfer the combined mixture into the prepared baking dish, spreading it evenly. Top the casserole with crispy fried onion strings to your liking. I like to be generous with the onion strings for a satisfying crunch and extra flavor.

Bake the assembled casserole in the preheated oven at 350°F (175°C) for 20 minutes, or until the casserole is heated through and the top is golden and slightly bubbly. Allow the casserole to cool for a few minutes before serving for neater portions.